Cardiac response classification using multiple classification windows

ABSTRACT

Methods and devices for classifying a cardiac response to pacing involve establishing a plurality of classification windows relative to and following a pacing pulse. One or more characteristics of a cardiac signal sensed following the pacing pulse are detected within one or more particular classification windows. The characteristics may be compared to one or more references. Classification of the cardiac response may be performed based on the comparison of the one or more characteristics to the one or more references and the particular classification windows in which the one or more characteristics are detected.

FIELD OF THE INVENTION

The present invention relates generally to implantable medical devicesand, more particularly, to classifying a cardiac response followingdelivery of a pace pulse.

BACKGROUND OF THE INVENTION

When functioning normally, the heart produces rhythmic contractions andis capable of pumping blood throughout the body. However, due to diseaseor injury, the heart rhythm may become irregular resulting in diminishedpumping efficiency. Arrhythmia is a general term used to describe heartrhythm irregularities arising from a variety of physical conditions anddisease processes. Cardiac rhythm management systems, such asimplantable pacemakers and cardiac defibrillators, have been used as aneffective treatment for patients with serious arrhythmias. These systemstypically comprise circuitry to sense electrical signals from the heartand a pulse generator for delivering electrical stimulation pulses tothe heart. Leads extending into the patient's heart are connected toelectrodes that contact the myocardium for sensing the heart'selectrical signals and for delivering stimulation pulses to the heart inaccordance with various therapies for treating the arrhythmias.

Cardiac rhythm management systems operate to stimulate the heart tissueadjacent to the electrodes to produce a contraction of the tissue.Pacemakers are cardiac rhythm management systems that deliver a seriesof low energy pace pulses timed to assist the heart in producing acontractile rhythm that maintains cardiac pumping efficiency. Pacepulses may be intermittent or continuous, depending on the needs of thepatient. There exist a number of categories of pacemaker devices, withvarious modes for sensing and pacing one or more heart chambers.

When a pace pulse produces a contraction in the heart tissue, theelectrical cardiac signal following the contraction is denoted thecaptured response (CR). The captured response may include an electricalsignal, denoted the evoked response signal, associated with the heartcontraction, along with a superimposed signal associated with residualpost pace polarization at the electrode-tissue interface. The magnitudeof the residual post pace polarization signal, or pacing artifact, maybe affected by a variety of factors including lead polarization,after-potential from the pace pulse, lead impedance, patient impedance,pace pulse width, and pace pulse amplitude, for example.

A pace pulse must exceed a minimum energy value, or capture threshold,to produce a contraction. It is desirable for a pace pulse to havesufficient energy to stimulate capture of the heart without expendingenergy significantly in excess of the capture threshold. Thus, accuratedetermination of the capture threshold is required for efficient paceenergy management. If the pace pulse energy is too low, the pace pulsesmay not reliably produce a contractile response in the heart and mayresult in ineffective pacing. If the pace pulse energy is too high, thepatient may experience discomfort and the battery life of the devicewill be shorter.

Capture detection allows the cardiac rhythm management system to adjustthe energy level of pace pulses to correspond to the optimum energyexpenditure that reliably produces a contraction. Further, capturedetection allows the cardiac rhythm management system to initiate aback-up pulse at a higher energy level whenever a pace pulse does notproduce a contraction.

At times, a pacing pulse may merge with an intrinsic beat, producing afusion beat. A fusion beat is a cardiac contraction that occurs when twocardiac depolarizations of a particular chamber, but from separateinitiation sites, merge. When the heart is being paced, a fusion beatmay occur when an intrinsic cardiac depolarization of a particularchamber merges with a pacer output pulse within that chamber. Fusionbeats, as seen on electrocardiographic recordings, exhibit variousmorphologies. The merging depolarizations of a fusion beat do notcontribute evenly to the total depolarization.

Pseudofusion occurs when a pacer output pulse is superimposed upon aspontaneous P wave during atrial pacing or upon a spontaneous QRScomplex during ventricular pacing. In pseudofusion, the pacing stimulusmay be ineffective because the tissue around the electrode has alreadyspontaneously depolarized and is in its refractory period.

During normal pacing, the presence of fusion or pseudofusion beats maybe of little consequence except for wasted energy due to the generationof unnecessary pace pulses. However, detection of fusion of pseudofusionbeats may be required during an automatic capture or thresholddetermination procedures. Fusion or pseudofusion beats may cause falsedetection of capture and may lead to erroneous capture threshold values.

Capture may be verified by detecting if a cardiac signal following apace pulse indicates a captured response. However, the captured responsemust be discerned from other responses, including the superimposedresidual post pace polarization without capture, intrinsic beats, andfusion/pseudofusion beats.

SUMMARY OF THE INVENTION

The present invention involves various methods and devices forclassifying cardiac responses to pacing stimulation. In accordance withone embodiment of the invention, a method of classifying a cardiacresponse to a pacing stimulation involves defining a plurality ofclassification windows relative and subsequent to a pacing stimulation.A cardiac signal following the pacing stimulation is sensed and acharacteristic of the cardiac signal is detected within a particularclassification window of the plurality of classification windows. Thecardiac response is classified based on the detected characteristic andthe particular classification window.

Another embodiment of the invention involves a method for determiningcardiac responses to pacing pulses. The method involves delivering asequence of pacing pulses to the heart. A plurality of classificationwindows are defined relative to and subsequent to each of the pacingpulses. Cardiac signals are sensed following the pacing pulses. One ormore characteristics of the cardiac signals are detected withinparticular classification windows. The detected cardiac signalcharacteristics s are compared to one or more references respectivelyassociated with types of cardiac pacing responses. The cardiac pacingresponses are classified based on the comparisons and the particularclassification windows in which the characteristics are detected.

Yet another embodiment of the invention involves a method forclassifying a cardiac pacing response. The method involves delivering apacing stimulation to the heart and defining a plurality ofclassification windows relative and subsequent to the pacingstimulation. A cardiac signal responsive to the pacing stimulation issensed and the peak of the sensed cardiac signal is detected within aparticular classification window. The cardiac response is determinedbased on the detected peak and the particular classification window.

In accordance with yet another embodiment of the invention, a medicaldevice for classifying a cardiac response to pacing includes a pacingpulse delivery circuit configured to deliver a pacing pulse to a heart.The medical device further includes a sensing circuit configured tosense a cardiac signal associated with the pacing pulse. A controlcircuit is coupled to the sensing circuit. The control circuit isconfigured to define a plurality of classification windows relative toand following the pacing pulse, detect a characteristic of the cardiacsignal sensed within a particular classification window, and classify acardiac response to the pacing pulse based on the detectedcharacteristic and the particular classification window.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial view of one embodiment of an implantable medicaldevice in accordance with embodiments of the invention;

FIG. 2A is a block diagram of an implantable medical device that may beused to classify a cardiac response to pacing in accordance withembodiments of the invention;

FIG. 2B is a schematic diagram of a circuit that may be used to generatepacing stimulations in accordance with embodiments of the invention;

FIG. 2C is a schematic diagram of a circuit that may be used to sense acardiac signal following the delivery of a pacing stimulation and toclassify the cardiac response to the pacing stimulation according toembodiments of the invention;

FIG. 3 is a graph illustrating a cardiac signal that indicates capture;

FIG. 4A illustrates superimposed graphs of a captured response andnon-captured and intrinsic response when the pacing pulse is deliveredon the RV rate channel and the cardiac signal following pacing is sensedon the RV shock channel in accordance with embodiments of the invention;

FIG. 4B illustrates superimposed graphs of a captured response and anon-captured response sensed using the same sensing vector in accordancewith embodiments of the invention;

FIG. 5 is a graph illustrating a propagation delay of a cardiac signalsensed on a shock channel following a pacing pulse delivered on a ratechannel;

FIG. 6 is a flowchart illustrating a method of classifying a cardiacresponse to pacing using multiple classification windows in accordancewith embodiments of the invention;

FIG. 7A illustrates establishment of a set of classification windowsrelative and subsequent to the pacing stimulation based on a capturedresponse template characteristic in accordance with embodiments of theinvention;

FIG. 7B is a flowchart illustrating a method of forming a capturedresponse (CR) template in accordance with embodiments of the invention;

FIG. 7C illustrates establishment of a set of classification windowsrelative and subsequent to the pacing stimulation based on an evokedresponse template characteristic in accordance with embodiments of theinvention;

FIG. 7D is a flowchart illustrating a method of providing an evokedresponse template for use in cardiac response classification inaccordance with embodiments of the invention;

FIG. 7E is a flowchart illustrating a method of acquiring a pacingartifact template in accordance with an embodiment of the invention;

FIG. 8A is a flowchart illustrating a method of cardiac responseclassification utilizing a captured response (CR) template to definemultiple classification windows in accordance with embodiments of theinvention;

FIG. 8B is a flowchart illustrating a method of cardiac responseclassification using an evoked response template to define multipleclassification windows in accordance with embodiments of the invention.

FIG. 9 is a diagram illustrating a cardiac signal sensed within multipleclassification windows established following a pacing pulse inaccordance with embodiments of the invention;

FIG. 10 is a flowchart illustrating a method for performing cardiacresponse classification in accordance with embodiments of the invention;

FIG. 11 is a diagram illustrating fusion, capture, and non-capture plusintrinsic response classification windows in accordance with embodimentsof the invention;

FIG. 12 is a flowchart illustrating a method of classifying a cardiacresponse using fusion, capture, and intrinsic classification windows inaccordance with an embodiment of the invention;

FIG. 13 is a diagram illustrating peak width classification referencesused to classify a cardiac response in accordance with embodiments ofthe invention;

FIGS. 14A and 14B are graphs illustrating the peak width of a capturedresponse and an intrinsic beat, respectively;

FIG. 15 is a flowchart of a method of classifying a cardiac responseusing peak width references according to embodiments of the invention;

FIG. 16 is a flowchart of a method of implementing a cardiac responseclassification process in accordance with the embodiments of theinvention;

FIG. 17 is a diagram illustrating the use of captured response andintrinsic beat templates in connection with classification of a cardiacresponse in accordance with embodiments of the invention;

FIG. 18 is a flowchart of a method of using template references toclassify a cardiac response in accordance with embodiments of theinvention.

FIGS. 19 and 20 illustrate positive and negative type fiducial pointsdetermined from rate channel signals in accordance with embodiments ofthe invention;

FIGS. 21 and 22 show morphological features, including turning point andflat slope features, respectively, for choosing template features inaccordance with embodiments of the invention; and

FIGS. 23 and 24 show morphological features, including turning point andflat slope features, respectively, for choosing template features inaccordance with embodiments of the present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail below. It is to be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings forming a part hereof, and inwhich are shown by way of illustration, various embodiments by which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

Embodiments of the invention are directed methods and devices forclassifying the cardiac response following the delivery of pacingstimulation to the heart. In accordance with various aspects of theinvention, cardiac response classification may be implemented bydefining a plurality of classification windows relative to and followinga pacing stimulation.

The pacing stimulation may be delivered to any heart chamber. Forexample, the pacing stimulation may be delivered to one of the rightventricle, the left ventricle, the right atrium, and the left atrium.

A cardiac signal following the pacing stimulation is sensed. One or morecharacteristics of the cardiac signal, for example, a peak, slope,curvature, sequence of feature points, or other characteristic of thecardiac signal are detected in one or more particular classificationwindows. The cardiac response to the pacing stimulation is determinedbased on the one or more detected characteristics and the one or moreparticular classification windows in which the one or morecharacteristics are detected. The cardiac response may be determined tobe one of a captured response, a non-captured response, a non-capturedresponse added to an intrinsic beat, a fusion/pseudofusion beat, andnoise, for example.

Various embodiments of the invention involve using the same electrodecombination for pacing and sensing. Other embodiments involve using anelectrode combination for pacing that is different from the electrodecombination used for sensing the cardiac response to pacing. Employingdifferent electrode combinations for pacing and sensing may enhancecardiac response classification. For example, using different electrodecombinations for pacing and sensing may facilitate detection offusion/pseudofusion beats. Further, such a configuration may be used toenhance discrimination of fusion/pseudofusion beats from captured beats.

By way of example, the processes of the present invention may be used toenhance capture threshold testing to determine the optimal energy forpacing. Determination of the optimal pacing energy may be implemented,for example, by an automatic capture threshold testing procedureexecuted by an implantable cardiac rhythm management system.Additionally, automatic capture verification may be used to monitorpacing on a beat-by-beat basis. Automatic capture verification may beused to control back up pacing when a pace pulse delivered to the heartfails to evoke a captured response (CR). These and other applicationsmay be enhanced by employment of the systems and methods of the presentinvention.

Those skilled in the art will appreciate that reference to a capturethreshold procedure indicates a method of determining the capturethreshold in one of left atrium, right atrium, left ventricle, and rightventricle. In such a procedure, the pacemaker, automatically or uponcommand, initiates a search for the capture threshold of the selectedheart chamber. The capture threshold is defined as the lowest pacingenergy that consistently captures the heart.

In one example of an automatic capture threshold procedure, thepacemaker delivers a sequence of pacing pulses to the heart and detectsthe cardiac responses to the pace pulses. The energy of the pacingpulses may be decreased in discrete steps until a predetermined numberof loss-of-capture events occur. The pacemaker may increase thestimulation energy in discrete steps until a predetermined number ofcapture events occur to confirm the capture threshold. A capturethreshold test may be performed using cardiac response classificationmethods of the present invention.

Other procedures for implementing capture threshold testing may beutilized. In one example, the pacing energy may be increased in discretesteps until capture is detected. In another example, the pacing energymay be adjusted according to a binomial search pattern.

Automatic capture threshold determination is distinguishable fromautomatic capture detection, a procedure that may occur on abeat-by-beat basis during pacing. Automatic capture detection verifiesthat a delivered pace pulse results in a captured response. When acaptured response is not detected following a pace pulse, the pacemakermay deliver a back up safety pace to ensure consistent pacing. The backup pace may be delivered, for example, about 70-80 ms after the initialpace pulse. If a predetermined number of pace pulses delivered duringnormal pacing do not produce a captured response, the pacemaker mayinitiate a capture threshold test to determine the capture threshold.Automatic capture detection and back up pacing may be implemented usingthe cardiac response classification processes of the present invention.

The embodiments of the present system illustrated herein are generallydescribed as being implemented in an implantable cardiac defibrillator(ICD) that may operate in numerous pacing modes known in the art.Various types of single and multiple chamber implantable cardiacdefibrillators are known in the art and may be used in connection withthe cardiac response classification methods of the present invention.The methods of the present invention may also be implemented in avariety of implantable or patient-external cardiac rhythm managementdevices, including single and multi chamber pacemakers, defibrillators,cardioverters, bi-ventricular pacemakers, cardiac resynchronizers, andcardiac monitoring systems, for example.

Although the present system is described in conjunction with animplantable cardiac defibrillator having a microprocessor-basedarchitecture, it will be understood that the implantable cardiacdefibrillator (or other device) may be implemented in any logic-basedintegrated circuit architecture, if desired.

Referring now to FIG. 1 of the drawings, there is shown a cardiac rhythmmanagement system that may be used to implement cardiac responseclassification methods of the present invention. The cardiac rhythmmanagement system in FIG. 1 includes an ICD 100 electrically andphysically coupled to a lead system 102. The housing and/or header ofthe ICD 100 may incorporate one or more electrodes 208, 209 used toprovide electrical stimulation energy to the heart and to sense cardiacelectrical activity. The ICD 100 may utilize all or a portion of the ICDhousing as a can electrode 209. The ICD 100 may include an indifferentelectrode positioned, for example, on the header or the housing of theICD 100. If the ICD 100 includes both a can electrode 209 and anindifferent electrode 208, the electrodes 208, 209 typically areelectrically isolated from each other.

The lead system 102 is used to detect electric cardiac signals producedby the heart 101 and to provide electrical energy to the heart 101 undercertain predetermined conditions to treat cardiac arrhythmias. The leadsystem 102 may include one or more electrodes used for pacing, sensing,and/or defibrillation. In the embodiment shown in FIG. 1, the leadsystem 102 includes an intracardiac right ventricular (RV) lead system104, an intracardiac right atrial (RA) lead system 105, an intracardiacleft ventricular (LV) lead system 106, and an extracardiac left atrial(LA) lead system 108. The lead system 102 of FIG. 1 illustrates oneembodiment that may be used in connection with the cardiac responseclassification methodologies described herein. Other leads and/orelectrodes may additionally or alternatively be used.

The lead system 102 may include intracardiac leads 104, 105, 106implanted in a human body with portions of the intracardiac leads 104,105, 106 inserted into a heart 101. The intracardiac leads 104, 105, 106include various electrodes positionable within the heart for sensingelectrical activity of the heart and for delivering electricalstimulation energy to the heart, for example, pacing pulses and/ordefibrillation shocks to treat various arrhythmias of the heart.

As illustrated in FIG. 1, the lead system 102 may include one or moreextracardiac leads 108 having electrodes, e.g., epicardial electrodes,positioned at locations outside the heart for sensing and pacing one ormore heart chambers.

The right ventricular lead system 104 illustrated in FIG. 1 includes anSVC-coil 116, an RV-coil 114, an RV-ring electrode 111, and an RV-tipelectrode 112. The right ventricular lead system 104 extends through theright atrium 120 and into the right ventricle 119. In particular, theRV-tip electrode 112, RV-ring electrode 111, and RV-coil electrode 114are positioned at appropriate locations within the right ventricle 119for sensing and delivering electrical stimulation pulses to the heart.The SVC-coil 116 is positioned at an appropriate location within theright atrium chamber 120 of the heart 101 or a major vein leading to theright atrial chamber 120 of the heart 101.

In one configuration, the RV-tip electrode 112 referenced to the canelectrode 209 may be used to implement unipolar pacing and/or sensing inthe right ventricle 119. Bipolar pacing and/or sensing in the rightventricle may be implemented using the RV-tip 112 and RV-ring 111electrodes. In yet another configuration, the RV-ring 111 electrode mayoptionally be omitted, and bipolar pacing and/or sensing may beaccomplished using the RV-tip electrode 112 and the RV-coil 114, forexample. The right ventricular lead system 104 may be configured as anintegrated bipolar pace/shock lead. The RV-coil 114 and the SVC-coil 116are defibrillation electrodes.

The left ventricular lead 106 includes an LV distal electrode 113 and anLV proximal electrode 117 located at appropriate locations in or aboutthe left ventricle 124 for pacing and/or sensing the left ventricle 124.The left ventricular lead 106 may be guided into the right atrium 120 ofthe heart via the superior vena cava. From the right atrium 120, theleft ventricular lead 106 may be deployed into the coronary sinusostium, the opening of the coronary sinus 150. The lead 106 may beguided through the coronary sinus 150 to a coronary vein of the leftventricle 124. This vein is used as an access pathway for leads to reachthe surfaces of the left ventricle 124 which are not directly accessiblefrom the right side of the heart. Lead placement for the leftventricular lead 106 may be achieved via subclavian vein access and apreformed guiding catheter for insertion of the LV electrodes 113, 117adjacent to the left ventricle.

Unipolar pacing and/or sensing in the left ventricle may be implemented,for example, using the LV distal electrode referenced to the canelectrode 209. The LV distal electrode 113 and the LV proximal electrode117 may be used together as bipolar sense and/or pace electrodes for theleft ventricle. The left ventricular lead 106 and the right ventricularlead 104, in conjunction with the ICD 100, may be used to providecardiac resynchronization therapy such that the ventricles of the heartare paced substantially simultaneously, or in phased sequence, toprovide enhanced cardiac pumping efficiency for patients suffering fromchronic heart failure.

The right atrial lead 105 includes a RA-tip electrode 156 and an RA-ringelectrode 154 positioned at appropriate locations in the right atrium120 for sensing and pacing the right atrium 120. In one configuration,the RA-tip 156 referenced to the can electrode 209, for example, may beused to provide unipolar pacing and/or sensing in the right atrium 120.In another configuration, the RA-tip electrode 156 and the RA-ringelectrode 154 may be used to effect bipolar pacing and/or sensing.

FIG. 1 illustrates one embodiment of a left atrial lead system 108. Inthis example, the left atrial lead 108 is implemented as an extracardiaclead with LA distal 118 and LA proximal 115 electrodes positioned atappropriate locations outside the heart 101 for sensing and pacing theleft atrium 122. Unipolar pacing and/or sensing of the left atrium maybe accomplished, for example, using the LA distal electrode 118 to thecan 209 pacing vector. The LA proximal 115 and LA distal 118 electrodesmay be used together to implement bipolar pacing and/or sensing of theleft atrium 122.

Referring now to FIG. 2A, there is shown an embodiment of a cardiacdefibrillator 200 suitable for implementing a cardiac responseclassification methodology of the present invention. FIG. 2A shows acardiac defibrillator divided into functional blocks. It is understoodby those skilled in the art that there exist many possibleconfigurations in which these functional blocks can be arranged. Theexample depicted in FIG. 2A is one possible functional arrangement.Other arrangements are also possible. For example, more, fewer ordifferent functional blocks may be used to describe a cardiacdefibrillator suitable for implementing the cardiac responseclassification methodology of the present invention. In addition,although the cardiac defibrillator 200 depicted in FIG. 2A contemplatesthe use of a programmable microprocessor-based logic circuit, othercircuit implementations may be utilized.

The cardiac defibrillator 200 depicted in FIG. 2A includes circuitry forreceiving cardiac signals from a heart and delivering electricalstimulation energy to the heart in the form of pacing pulses ordefibrillation shocks. In one embodiment, the circuitry of the cardiacdefibrillator 200 is encased and hermetically sealed in a housing 201suitable for implanting in a human body. Power to the cardiacdefibrillator 200 is supplied by an electrochemical battery 280. Aconnector block (not shown) is attached to the housing 201 of thecardiac defibrillator 200 to allow for the physical and electricalattachment of the lead system conductors to the circuitry of the cardiacdefibrillator 200.

The cardiac defibrillator 200 may be a programmable microprocessor-basedsystem, including a control system 220 and a memory 270. The memory 270may store parameters for various pacing, defibrillation, and sensingmodes, along with other parameters. Further, the memory 270 may storedata indicative of cardiac signals received by other components of thecardiac defibrillator 200. The memory 270 may be used, for example, forstoring historical EGM and therapy data. The historical data storage mayinclude, for example, data obtained from long term patient monitoringused for trending or other diagnostic purposes. Historical data, as wellas other information, may be transmitted to an external programmer unit290 as needed or desired.

The control system 220 and memory 270 may cooperate with othercomponents of the cardiac defibrillator 200 to control the operations ofthe cardiac defibrillator 200. The control system depicted in FIG. 2Aincorporates a cardiac response classification processor 225 forclassifying cardiac responses to pacing stimulation in accordance withvarious embodiments of the present invention. The control system 220 mayinclude additional functional components including a pacemaker controlcircuit 222, an arrhythmia detector 221, and a template processor 224,along with other components for controlling the operations of thecardiac defibrillator 200.

Telemetry circuitry 260 may be implemented to provide communicationsbetween the cardiac defibrillator 200 and an external programmer unit290. In one embodiment, the telemetry circuitry 260 and the programmerunit 290 communicate using a wire loop antenna and a radio frequencytelemetric link, as is known in the art, to receive and transmit signalsand data between the programmer unit 290 and the telemetry circuitry260. In this manner, programming commands and other information may betransferred to the control system 220 of the cardiac defibrillator 200from the programmer unit 290 during and after implant. In addition,stored cardiac data pertaining to capture threshold, capture detectionand/or cardiac response classification, for example, along with otherdata, may be transferred to the programmer unit 290 from the cardiacdefibrillator 200.

In the embodiment of the cardiac defibrillator 200 illustrated in FIG.2A, electrodes RA-tip 156, RA-ring 154, RV-tip 112, RV-ring 111,RV-coil, SVC-coil, LV distal electrode 113, LV proximal electrode 117,LA distal electrode 118, LA proximal electrode 115, indifferentelectrode 208, and can electrode 209 are coupled through a switch matrix210 to sensing circuits 231-237.

A right atrial sensing circuit 231 serves to detect and amplifyelectrical signals from the right atrium of the heart. Bipolar sensingin the right atrium may be implemented, for example, by sensing voltagesdeveloped between the RA-tip 156 and the RA-ring 154. Unipolar sensingmay be implemented, for example, by sensing voltages developed betweenthe RA-tip 156 and the can electrode 209. Outputs from the right atrialsensing circuit are coupled to the control system 220.

A right ventricular sensing circuit 232 serves to detect and amplifyelectrical signals from the right ventricle of the heart. The rightventricular sensing circuit 232 may include, for example, a rightventricular rate channel 233 and a right ventricular shock channel 234.Right ventricular cardiac signals sensed through use of the RV-tip 112electrode are right ventricular near-field signals and are denoted RVrate channel signals. A bipolar RV rate channel signal may be sensed asa voltage developed between the RV-tip 112 and the RV-ring.Alternatively, bipolar sensing in the right ventricle may be implementedusing the RV-tip electrode 112 and the RV-coil 114. Unipolar ratechannel sensing in the right ventricle may be implemented, for example,by sensing voltages developed between the RV-tip 112 and the canelectrode 209.

Right ventricular cardiac signals sensed through use of the RV-coilelectrode 114 are far-field signals, also referred to as RV morphologyor RV shock channel signals. More particularly, a right ventricularshock channel signal may be detected as a voltage developed between theRV-coil 114 and the SVC-coil 116. A right ventricular shock channelsignal may also be detected as a voltage developed between the RV-coil114 and the can electrode 209. In another configuration the canelectrode 209 and the SVC-coil electrode 116 may be electrically shortedand a RV shock channel signal may be detected as the voltage developedbetween the RV-coil 114 and the can electrode 209/SVC-coil 116combination.

Outputs from the right ventricular sensing circuit 232 are coupled tothe control system 220. In one embodiment of the invention, rate channelsignals and shock channel signals may be used to develop morphologytemplates for analyzing cardiac signals. In this embodiment, ratechannel signals and shock channel signals may be transferred from theright ventricular sensing circuit 232 to the control system 220 and to atemplate processor 224 where the morphological characteristics of acardiac signal are analyzed. The template processor 224 works incombination with the control system 220 and the memory 270 to generateand maintain various types of templates, including, for example,templates used for arrhythmia discrimination as well as cardiac responseclassification as described in more detail below.

Left atrial cardiac signals may be sensed through the use of one or moreleft atrial electrodes 115, 118, which may be configured as epicardialelectrodes. A left atrial sensing circuit 235 serves to detect andamplify electrical signals from the left atrium of the heart. Bipolarsensing and/or pacing in the left atrium may be implemented, forexample, using the LA distal electrode 118 and the LA proximal electrode115. Unipolar sensing and/or pacing of the left atrium may beaccomplished, for example, using the LA distal electrode 118 to canvector 209 or the LA proximal electrode 115 to can vector 209.

A left ventricular sensing circuit 236 serves to detect and amplifyelectrical signals from the left ventricle of the heart. Bipolar sensingin the left ventricle may be implemented, for example, by sensingvoltages developed between the LV distal electrode 113 and the LVproximal electrode 117. Unipolar sensing may be implemented, forexample, by sensing voltages developed between the LV distal electrode113 or the LV proximal electrode 117 to the can electrode 209.

Optionally, an LV coil electrode (not shown) may be inserted into thepatient's cardiac vasculature, e.g., the coronary sinus, adjacent theleft heart. Signals detected using combinations of the LV electrodes,113, 117, LV coil electrode (not shown), and/or can electrodes 209 maybe sensed and amplified by the left ventricular sensing circuitry 236.The output of the left ventricular sensing circuit 236 is coupled to thecontrol system 220.

The outputs of the switching matrix 210 may be operated to coupleselected combinations of electrodes 111, 112, 113, 114, 115, 116, 117,118, 156, 154 to an evoked response sensing circuit 237. The evokedresponse sensing circuit 237 serves to sense and amplify voltagesdeveloped using various combinations of electrodes for cardiac responseclassification in accordance with embodiments of the invention.

In the embodiments described below, various combinations of pacing andsensing electrodes may be utilized in connection with pacing and sensingthe cardiac signal following the pace pulse to classify the cardiacresponse to the pacing pulse. For example, in some embodiments, a firstelectrode combination is used for pacing a heart chamber and a secondelectrode combination is used to sense the cardiac signal followingpacing. In other embodiments, the same electrode combination is used forpacing and sensing.

Sensing the cardiac signal following a pacing pulse using the sameelectrode combination for both pacing and sensing may yield a sensedcardiac signal including a pacing artifact component associated withresidual post pace polarization at the electrode-tissue interface. Thepacing artifact component may be superimposed on a smaller signalindicative of the cardiac response to the pacing pulse, i.e., the evokedresponse. The pacing output circuitry may include a coupling capacitorto block DC components from the heart and to condition the pacingstimulus pulse. A relatively large coupling capacitor may cause a largerpacing artifact that decays exponentially over a relatively long periodof time.

The presence of a large pacing artifact signal may complicate theclassification of the cardiac response to pacing. Various embodiments ofthe invention are directed to methods involving detection of a cardiacsignal following pacing and canceling the pacing artifact from thedetected signal. Classification of the cardiac response to pacing isimplemented using the pacing artifact cancelled signal. Cancellation ofthe pacing artifact in cardiac response classification is particularlyimportant when the same or similar electrode combinations are used bothfor delivering pacing pulses and for sensing the cardiac signalsfollowing the delivery of the pacing pulses. Cancellation of the pacingartifact may also be used when a first electrode combination is used forpacing the heart chamber and a different electrode combination is usedto sense the subsequent cardiac response.

In various embodiments described herein a first electrode combinationmay be used for pacing the heart chamber and a second electrodecombination used for sensing the cardiac signals following the pace forcardiac response classification. If different electrode combinations areused for pacing and sensing, a temporal separation between the cardiacresponse signal, e.g., the evoked response, and the pacing artifact mayfacilitate classification of the cardiac response to pacing. Thetemporal separation occurs due to the propagation delay of thedepolarization wavefront initiated at the pacing electrode and travelingto a sensing electrode that is physically spaced apart from the pacingelectrode. The temporal separation of the cardiac response signal andthe pacing artifact may be sufficient to obviate cancellation of thepacing artifact. Use of different electrodes for pacing and sensing inconnection with capture verification is described in commonly owned U.S.Pat. No. 6,128,535 which is incorporated herein by reference.

The pacemaker control circuit 222, in combination with pacing circuitryfor the left atrium, right atrium, left ventricle, and right ventricle241, 242, 243, 244, may be implemented to selectively generate anddeliver pacing pulses to the heart using various electrode combinations.The pacing electrode combinations may be used to effect bipolar orunipolar pacing of the heart chambers as described above.

As described above, bipolar or unipolar pacing pulses may be deliveredto a heart chamber using one of the pacing vectors as described above.The electrical signal following the delivery of the pacing pulses may besensed through various sensing vectors coupled through the switch matrix210 to the evoked response sensing circuit 237 and used to classify thecardiac response to pacing.

In one example, the cardiac signal following the pacing pulse may besensed using the same vector as was used for delivery of the pacingpulse. In this scenario, the pacing artifact may be canceled from thesensed cardiac signal using the pacing artifact cancellation techniquesdescribed below. Following cancellation of the pacing artifact, multiplecardiac response classification windows may be defined following thepacing pulse and used to classify the cardiac response to pacing. Thecardiac response may be classified as one of a captured response, anon-captured response, a non-captured response and an intrinsic beat, afusion/pseudofusion beat, and noise, for example.

In another example, the vector used to sense the cardiac signalfollowing the pacing pulse may be different from the vector that wasused to deliver the pacing pulse. The sensing vector may be selected tominimize the pacing artifact. Cancellation of the pacing artifact maynot be necessary if the pacing artifact is sufficiently minimized usingthis technique.

In one embodiment, the cardiac signal sensed using a sensing vectordifferent from the pacing vector may be used to detectfusion/pseudofusion beats. In another embodiment, the cardiac signalsensed using a sensing vector different from the pacing vector may beused to discriminate between fusion/pseudofusion beats and capturedbeats. In other embodiments, the cardiac response to the pacingstimulation may be classified as one of a captured response, anon-captured response, a non-captured response and an intrinsic beat, afusion/pseudofusion beat, and noise, for example. In variousembodiments, the pacing pulse may be delivered using electrodesassociated with a near-field vector and the sensing vector may be afar-field vector. In an example of right ventricular pacing and cardiacresponse sensing, the pacing vector may be the rate channel vector andthe sensing vector may be the shock channel vector. Cardiac responseclassification may be accomplished, for example, using multipleclassification windows defined following delivery of the pacing pulse asdescribed in greater detail below.

Possible sensing vectors for effecting cardiac response classificationmay include, for example, RV-tip 112 and RV-coil 114, RV-coil 114 and LVdistal electrode 113, RV coil 114 and LV proximal electrode 117, RV-coil114 and can 209, RV-coil 114 and SVC coil 116, RV-coil 114 and SVC coil116 tied and the can 209, RV-coil 114 and A-ring 154, RV-coil 114 andRA-tip 156, LV distal electrode 113 and LV proximal electrode 117, LVdistal electrode 113 and can 209, LV distal electrode 113 and SVC coil116, LV distal electrode 113 and A-ring 154, LV distal electrode 113 andA-tip 156, LV proximal electrode 117 and can 209, LV proximal electrode117 and SVC coil 116, LV proximal electrode 117 and A-ring 154, LVproximal electrode 117 and RA-tip 156, SVC coil 116 and can 209, RA-ring154 and can 209, RA-tip 156 and can 209, SVC coil 116 and A-ring 154,SVC coil 116 and RA-tip 156 and RA-ring 154 and RA-tip 156. This list isnot exhaustive and other sensing vector combinations may be developed toimplement cardiac response classification in accordance with embodimentsof the invention. For example, other combinations may include a coronarysinus electrode, an indifferent electrode, a leadless ECG electrode,cardiac epicardial electrodes, subcutaneous electrodes, and/or otherelectrodes.

Approaches for using leadless ECG electrodes for capture detection aredescribed in U.S. Pat. No. 5,222,493, which is incorporated by referencein its entirety.

Subcutaneous electrodes may provide additional sensing vectors useablefor cardiac response classification. In one implementation, cardiacrhythm management system may involve a hybrid system including a firstdevice, e.g. a pacemaker coupled to an intracardiac lead system,configured to pace the heart, and a second device, e.g. a defibrillatorcoupled to a subcutaneous lead system, configured to perform functionsother than pacing. The second device may be employed to detect andclassify cardiac responses to pacing based on signals sensed usingsubcutaneous electrode arrays. The first and second devices may operatecooperatively with communication between the devices occurring over awireless link, for example. Examples of subcutaneous electrode systemsand devices are described in commonly owned U.S. patent application Ser.No. 10/462,001, filed Jun. 13, 2003 and Ser. No. 10/465,520, filed Jun.19, 2003, which are incorporated herein by reference in their respectiveentireties.

For right ventricular pacing, bipolar pacing may be delivered using theRV-tip electrode 112 and the RV-ring electrode 111. Unipolar pacing maybe delivered using the RV-tip 112 to can 209 vector. The preferredsensing electrode combinations for cardiac response classificationfollowing RV pacing include RV-coil 114 to SVC-coil 116 tied to the canelectrode 209, RV-coil 114 to can electrode 209, and, if the systemincludes an left ventricular lead, LV distal electrode 113 to LVproximal electrode 117.

In an example of left ventricular pacing, bipolar pacing pulses may bedelivered to the left ventricle between the LV distal electrode 113 andthe LV proximal electrode 117. In another example, unipolar pacingpulses may be delivered to the left ventricle, for example, between theLV distal electrode 113 and the can 209. The cardiac signal followingthe delivery of the pacing pulses may preferably be sensed using the LVproximal electrode 117 and the can 209.

In an example of right atrial pacing, bipolar pacing pulses may bedelivered to the right atrium between the RA-tip electrode 156 and theRA-ring electrode 154. In another example, unipolar pacing pulses may bedelivered to the right atrium, for example, between the RA-tip electrode156 and the can electrode 209. For unipolar right atrial pacing, thepreferred electrode combination for sensing cardiac signals followingpacing for cardiac response classification comprises the RA-ring 154 toindifferent electrode.

In an example of left atrial pacing, bipolar pacing pulses may bedelivered to the left atrium between the LA distal electrode 118 and theLA proximal electrode 115. In another example, unipolar pacing pulsesmay be delivered to the left atrium, for example, between the LA distalelectrode 118 and the can electrode 209. The cardiac signal followingthe delivery of the pacing pulses and used for cardiac responseclassification may preferably be sensed using the RA-tip 156 to RA-ring154 vector.

In one embodiment of the invention, a switching matrix 210 is coupled tothe RA-tip 156, RA-ring 154, RV-tip 112, RV-coil 114, LV distalelectrode 113, LV proximal electrode 117, SVC coil 116, LA distalelectrode 118, LA proximal electrode 115, indifferent, and can 209electrodes. The switching matrix 210 may be arranged to provideconnections to various configurations of pacing and defibrillationelectrodes. The outputs of the switching matrix 210 are coupled to anevoked response (ER) sensing circuit 237 that serves to sense andamplify cardiac signals detected between the selected combinations ofelectrodes. The detected signals are coupled through the ER amplifier237 to a cardiac response classification processor 225. The cardiacresponse classification processor 225 includes circuitry configured toclassify a cardiac response to a pacing stimulation, including, forexample, classifying a captured response, a non-captured response, anintrinsic beat added to a non-captured response, and afusion/pseudofusion response, in accordance with the invention.

FIGS. 2B and 2C illustrate more detailed examples of pacing and sensingcircuitry, respectively, that may be used for cardiac pace/sensechannels of a pacemaker in accordance with embodiments of the invention.It will be appreciated that the example pacing and sensing circuitsillustrated in FIGS. 2B and 2C may be arranged to achieve the pacing andsensing vectors described above.

In example embodiments of the invention, the pacing circuit of FIG. 2Bincludes a power supply or battery 261, a first switch 262, a secondswitch 264, a pacing charge storage capacitor 263, coupling capacitor265, and a pacer capacitor charging circuit 269 all of which arecooperatively operable under the direction of a controller of knownsuitable construction. The power supply or battery 261 is preferably thebattery provided to power the pacemaker and may comprise any number ofcommercially available batteries suitable for pacing applications. Theswitches 262, 264 may be implemented using any number of conventionallyavailable switches. The pacing capacitor charging circuit 269 includescircuitry to regulate the voltage across the pacing charge storagecapacitor 263.

The pacing charge storage capacitor 263 may also comprise any number ofconventional storage capacitors that can be Used to develop a sufficientpacing charge for stimulating the heart. The primary function of thecoupling capacitor 265 is to block any DC signal from reaching the heartduring pacing and additionally to attenuate the polarization voltage or“afterpotential” that results from pacing. The coupling capacitor 265may have a capacitance, for example, in the range of about 2 microfaradsto about 22 microfarads. Energy stored in the pacing charge storagecapacitor 263 may be delivered to the heart 268 using variouscombinations of cardiac electrodes 266, 267, as described above.

FIG. 2C illustrates a block diagram of circuit 295 that may be used tosense cardiac signals following the delivery of a pacing stimulation andclassify the cardiac response to the pacing stimulation according toembodiments of the invention. A switch matrix 284 is used to couple thecardiac electrodes 271, 272 in various combinations discussed above tothe sensing portion 270 of the cardiac response classification circuit295. The sensing portion 270 includes filtering and blanking circuitry275, 277, sense amplifier 285, band pass filter 281, and analog todigital converter 282. The analog to digital converter 282 is coupled toa cardiac response classification processor 283.

A control system, e.g., the control system 220 depicted in FIG. 2A, isoperatively coupled to components of the cardiac response classificationcircuit 280 and controls the operation of the cardiac responseclassification circuit 295, including the filtering and blankingcircuits 275, 277. Following a blanking period of sufficient durationfollowing delivery of the pacing stimulation, the blanking circuitry275, 277 operates to allow detection of a cardiac signal responsive tothe pacing stimulation. The cardiac signal is filtered, amplified, andconverted from analog to digital form. The digitized signal iscommunicated to the cardiac response classification processor 283 whichoperates in cooperation with other components of the control system 220,FIG. 2A, including the template processor 241, FIG. 2A, to classifycardiac responses to pacing according to embodiments of the invention.

When pacing pulses delivered to the heart produce a depolarization wavein cardiac tissue resulting in a cardiac contraction, a capturedresponse may be detected by examining the cardiac signal following thedelivery of the pacing pulse. FIG. 3 is a graph illustrating the outputof the sensing portion 270 of the cardiac response classificationcircuit 295 of FIG. 2C in which the cardiac signal consistentlyindicates capture following a sequence of pacing pulses. In thisexample, a pacing pulse is delivered to the heart using the RV-tip andRV-coil electrodes, also referred to herein as a right ventricular ratechannel. The cardiac signal following a right ventricular pace is sensedusing a RV-coil to SVC-coil+can sensing vector, also referred to hereinas the shock channel. FIG. 4A provides superimposed graphs of a capturedresponse 430 and non-captured and intrinsic response 420 when the pacingpulse is delivered on the RV rate channel and the cardiac signalfollowing pacing is sensed on the RV shock channel.

In another example, the same vector may be used to pace the heartchamber and sense the cardiac signal following the pace to classify thecardiac response. Pacing in the right ventricle may be accomplishedusing the pacing vector RV-tip to RV-ring, for example. FIG. 4Billustrates superimposed graphs of a captured response 440 and anon-captured response 450 sensed using that same vector, e.g., RA-tip toRA-ring.

As previously discussed, if a first vector, e.g., rate channel vectorRV-tip to RV-coil, is used to deliver a pacing pulse and a secondvector, e.g., shock channel vector RV-coil to SVC-coil or RV-coil toSVC-coil+can, is used to sense the cardiac signal responsive to thepacing pulse, the pacing artifact is separated from the evoked responsedue to a propagation delay from RV-tip to RV-coil. FIG. 5 is a graphillustrating a signal 520 sensed on a right ventricular (RV) shockchannel vector following a pacing pulse 510 delivered on a rate channel.The cardiac signal 520 exhibits a propagation delay 530, for example, apropagation delay of about 55 ms, between the pacing pulse 510 and theportion of the cardiac signal indicating a captured response 540.

FIG. 6 is a flowchart illustrating a method of classifying a cardiacresponse to a pacing stimulation in accordance with embodiments of theinvention. In this method the same electrode combination may be used forpacing and sensing, or a first electrode combination may be used forpacing and a second electrode combination may be used for sensing. Ifthe same electrode combination is used for pacing and sensing, thenpacing artifact cancellation may facilitate cardiac responseclassification. In accordance with this method, a plurality of responseclassification windows are defined 610 subsequent to delivery of apacing pulse. A cardiac signal following the pacing pulse is sensed 620.One or more characteristics of the cardiac signal, for example,magnitude, slope or sequence of morphological feature points, aredetected 630 in one or more particular classification windows of theplurality of classification windows. The cardiac response is classified640 based on the one or more detected characteristics and the one ormore particular classification windows in which the characteristic isdetected. Although the flowchart of FIG. 6 describes classification of acardiac response based on the detection of a characteristic of thecardiac signal within a particular classification window, any number ofcharacteristics of the cardiac signal detected in any number of theclassification windows may be used to classify the cardiac responseaccording to the principles of invention.

Classification of a cardiac response to pacing may be accomplished usinga multiple classification window approach. This approach may beapplicable for pacing and sensing using the same vector or pacing andsensing using different vectors. Classification of a cardiac response topacing in accordance with embodiments of the invention involvesanalyzing one or more features of the cardiac signal sensed following apacing stimulation with respect to multiple classification windows. Thecardiac response to pacing may be determined based on a feature of thecardiac signal and the classification window in which the feature isdetected. Although in various examples provided herein, theclassification windows are contiguous and non-overlapping, theclassification windows may be overlapping and/or may involve a delayinterval defined between classification windows.

FIG. 7A illustrates the establishment of a set of classification windowsrelative and subsequent to the pacing stimulation. In this example,three classification windows 720, 730, 740 are established based on aselected characteristic of a captured response template. A capturedresponse (CR) template exemplifies a waveform representative of acaptured response as illustrated in FIGS. 4A and 4B. The CR template maybe derived from a waveform that is produced when a pacing pulse capturesthe heart, and may include both the evoked response and the superimposedpacing artifact. A CR template may comprise, for example, a sequence ofsamples or feature points of a cardiac signal representing a capturedresponse. Multiple cardiac response classification windows may bedefined based on features of the CR template.

Initial generation of a CR template may be implemented by deliveringpacing pulses to the heart at an energy greater than the capturethreshold. Delivery of pacing pulses at a high energy level may beperformed, for example, during a capture threshold test. A capturethreshold test may involve pacing a selected heart chamber at aninitially high energy level and ramping down the pacing energy untilloss of capture is detected. Pacing pulses delivered early in thecapture threshold test have energy levels exceeding the capturethreshold, and produce cardiac signals indicative of captured beats. Thepacing pulses may be delivered using a first vector and the cardiacsignals following pacing may be sensed using a second vector.Alternatively, pacing and sensing may be implemented using the samevector. Cardiac signals representing one of more captured cardiac beatsmay be used to form the CR template.

FIG. 7B is a flowchart illustrating a method of forming a CR template inaccordance with embodiments of the invention. Pacing pulses aredelivered 760 to a heart chamber at a pacing energy exceeding thecapture threshold for the chamber. The cardiac signal following deliveryof the pacing pulse is sensed 765. If the sensed cardiac signal is thefirst acquired signal 766, the cardiac signal is used 768 to form aninitial CR template. If the sensed cardiac signal is not the firstacquired signal 766, then the sensed cardiac signal is compared 770 tothe existing CR template. If the sensed cardiac signal is consistentwith 770 the CR template, then it is combined 775 with the CR template.A cardiac signal may be considered to be consistent with a template ifthe features, samples, or other morphological characteristics of thecardiac signal are determined to be sufficiently similar to the templatefeatures, samples, or morphological characteristics. If a cardiac signalis sufficiently similar to a template representative of a particulartype of cardiac beat, then the cardiac signal may be classified as theparticular type of beat. Various techniques may be used to compare atemplate and a cardiac signal, including the correlation techniquesdescribed herein.

In some implementations, a cardiac signal that is consistent 770 withthe CR template may be combined with the CR template by averaging thecardiac signal and the CR template sample by sample, or by otheraveraging methods. In other implementations, different methods ofcombining the cardiac signal with the template may be used. If morebeats are available 780 for CR template generation then the process ofblocks 760-775 is repeated. If no more beats are available for CRtemplate generation, then the CR template generation process is complete785.

In one implementation, the comparison between an existing CR templateand a sensed cardiac signal may be accomplished by calculating acorrelation coefficient (CC) comparing the sensed cardiac signal and theCR template using a technique such as Correlation Waveform Analysis(CWA). According to this technique, a correlation coefficient (CC) maybe calculated to compare the sensed cardiac signal to the CR templatesample by sample. In one particular embodiment, Equation 1 is used tocompute the CC between the samples of a cardiac signal sensed followinga pacing pulse and the CR template samples.

$\begin{matrix}{{CC} = \frac{{N{\sum\limits_{i = 1}^{N}\;{X_{i}Y_{i}}}} - {\left( {\sum\limits_{i = 1}^{N}\; X_{i}} \right)\left( {\sum\limits_{i = 1}^{N}\; Y_{i}} \right)}}{\sqrt{\left( {{N{\sum\limits_{i = 1}^{N}\; X_{i}^{2}}} - \left( {\sum\limits_{i = 1}^{N}\; X_{i}} \right)^{2}} \right)\left( {{N{\sum\limits_{i = 1}^{N}\; Y_{i}^{2}}} - \left( {\sum\limits_{i = 1}^{N}\; Y_{i}} \right)^{2}} \right)}}} & \lbrack 1\rbrack\end{matrix}$

where, X_(i) represents template N samples and Y_(i) represents cardiacsignal N samples in this illustrative example. Typically, the number ofsamples associated with each waveform or template is about 33 samples.If the correlation coefficient is greater than a predetermined value,for example, about 0.71, the cardiac signal is considered to represent acaptured response signal and may be combined with the CR template.

In another implementation, features used to form an existing CR templateand features of a sensed cardiac signal may be compared by calculating afeature correlation coefficient (FCC). The FCC may be determined, forexample, using every fourth sample of the cardiac signal and thecaptured response template. For example, Equation 2, provided below, maybe used to compute the FCC between selected CR template features andcardiac signal features:

$\begin{matrix}{{FCC} = \frac{\left( {{N{\sum\limits_{i = 1}^{N}\;{X_{i}Y_{i}}}} - {\left( {\sum\limits_{i = 1}^{N}\; X_{i}} \right)\left( {\sum\limits_{i = 1}^{N}\; Y_{i}} \right)}} \right)^{2}}{\left( {{N{\sum\limits_{i = 1}^{N}\; X_{i}^{2}}} - \left( {\sum\limits_{i = 1}^{N}\; X_{i}} \right)^{2}} \right)\left( {{N{\sum\limits_{i = 1}^{N}\; Y_{i}^{2}}} - \left( {\sum\limits_{i = 1}^{N}\; Y_{i}} \right)^{2}} \right)}} & \lbrack 2\rbrack\end{matrix}$where, X_(i) represents CR template N features and Y_(i) represents beatN features. The sign of the numerator term is checked before squaring.If the numerator is negative, the beat is uncorrelated, and theremainder of the computation need not be performed.

If the FCC is greater than a predetermined value, for example 0.94, thenthe cardiac beat is correlated to the CR template. If the FCC is lessthan or equal to the predetermined value, then the cardiac beat isuncorrelated to the template.

The CR template may be periodically updated using cardiac signalsclassified as captured responses. Updating the CR template allows the CRtemplate to adapt to slow variations in the patient's captured responseover time. Updating the CR template may be accomplished by averaging, orotherwise combining, the samples or feature points of an existing CRtemplate with corresponding samples or feature points of cardiac signalsrepresenting captured response beats.

If the CR template is updated, the classification windows based on CRtemplate features or morphology may also be updated. For example, thetiming of a classification window based on a CR template feature may bemodified to accommodate an updated timing of the CR template feature.Further, the duration of one or more of the classification windows maybe modified based on updated information with respect to the CR templatemorphology.

The classification windows may be adapted based on statistics ofvariability of the captured response morphology and/or intrinsic beatmorphology, for example. In one implementation, captured responsesdetected during a capture threshold test and/or intrinsic beats may beused to form a statistical database for classification windowadaptation. The timing and/or duration of one or more of theclassification windows may be adjusted in response to the variability ofthe features of the captured response and/or the intrinsic response. Forexample, the timing and/or duration of a classification window may beset adaptively based on the statistics of variability of the timing ofthe intrinsic beat peak or the timing of the captured response peak.

In one implementation, a CR template may be formed or updated during acapture threshold test. The test may deliver pacing pulses to the heartat an initially high pacing energy and ramp down the pacing energy overa series of pulses until a loss of capture is detected. A CR templatemay be formed or updated using the cardiac signals associated withcaptured responses following delivery of high energy pace pulses to theheart during capture threshold testing.

Returning to FIG. 7A, establishment of classification windows used toclassify a cardiac signal following a pacing pulse is further described.In this example, classification windows may be established based on afeature or features of the captured response (CR) template. Asillustrated in FIG. 7A, a first classification window 730 may beestablished based on the time 750 of a selected characteristic of acaptured response (CR) template. In one example, the firstclassification window 730 is established based on the timing of the peakof the CR template, although other characteristics such as slope,curvature, amplitude, rise time or fall time may be used. The firstclassification window 730 represents a time interval defined in relationto the timing of the selected characteristic of the CR template 750 froma pacing stimulation 710. In the example illustrated in FIG. 7A, thefirst classification window 730 comprises a time interval, e.g., a timeinterval of about 20 ms, centered at the time 750 of the selected CRtemplate characteristic with respect to the time of the delivery of thepacing pulse.

In this example, a second classification window 720 may be definedsubsequent to the time 710 of the delivery of the pacing pulse and priorto the beginning of the first classification window 730. A thirdclassification window 740 may be defined following the end of the firstclassification window 730.

The classification windows may be defined for example, following ablanking period 760 that is initiated subsequent to the delivery 710 ofthe pacing pulse. The blanking period 760 may comprise an interval ofless than about 40 ms, or other value, for example. The first, secondand third classification windows may comprise a total time interval ofless than about 200 ms, for example.

FIGS. 7A and 7B and the associated discussion illustrate methods ofdefining classification windows using a captured response (CR) templatecharacteristic, e.g., the CR template peak in accordance withembodiments of the invention. Such a technique may be particularlyuseful if different pacing and sensing vectors are used to reduce theeffect of the pacing artifact. Classification of a cardiac responseusing multiple classification windows may be enhanced using cancellationof the pacing artifact, particularly if the same vector is used forpacing and sensing.

FIGS. 7C and 7D illustrate a method of cardiac response classificationinvolving cancellation of the pacing artifact from the sensed cardiacsignal in accordance with various embodiments of the invention.According to this approach, a pacing artifact template representative ofthe pacing artifact is determined. The captured response template may bedetermined as described above. The pacing artifact template is thencancelled from the captured response template. Cancellation of thepacing artifact template from the captured response template defines atemplate representative of the evoked response (ER), i.e., the portionof the cardiac signal representing the evoked response without thesuperimposed pacing artifact. Multiple cardiac response classificationwindows may be defined based on a feature or features of the ERtemplate.

For a paced beat, classification of a cardiac response to the pacingstimulation involves canceling the pacing artifact template from thecardiac signal sensed following a pacing pulse. One or more features ofthe pacing artifact cancelled signal may be analyzed with respect to themultiple classification windows. The cardiac response may be determinedbased on a feature of the pacing artifact cancelled cardiac signal andthe classification window in which the feature is detected.

As illustrated in FIG. 7C, a first classification window 735 may beestablished based on the time 755 of a selected characteristic of anevoked response (ER) template. In one example, the first classificationwindow 735 is established based on a peak of the ER template, althoughother characteristics such as slope, curvature, amplitude, rise time orfall time may be used. The first classification window 735 represents atime interval defined in relation to the timing of the selectedcharacteristic of the ER template 755 from a pacing stimulation 715. Inthe example illustrated in FIG. 7C, the first classification window 735comprises a time interval, e.g., a time interval of about 20 ms,centered at the time 755 of the selected ER template characteristic.

In this example, a second classification window 725 is establishedsubsequent to the time 715 of the delivery of the pacing pulse and priorto the beginning of the first classification window 735. A thirdclassification window 745 may be established following the end of thefirst classification window 735.

FIG. 7D is a flowchart illustrating a method of providing an evokedresponse template for use in cardiac response classification inaccordance with embodiments of the invention. In this example, the heartis stimulated by pace pulses having a voltage greater than the capturethreshold. The resultant captured cardiac responses are sensed andaveraged to form a captured response template. A pacing artifacttemplate is subtracted or otherwise cancelled from the captured responsetemplate to produce an evoked response template.

Turning to the flowchart of FIG. 7D, a pacing artifact template isprovided at block 786. Generation of the pacing artifact template isdescribed in connection with FIG. 7E below. A captured response templatemay be determined 787 by delivering a predetermined number of pacepulses at a pacing voltage greater than the capture threshold asdescribed in connection with FIG. 7B above. The pacing artifact templatemay be normalized 792 with respect to the captured response template.Following normalization, the pacing artifact template is canceled fromthe captured response template 793. For example, the pacing artifacttemplate may be canceled by subtracting the pacing artifact templatefrom the captured response template sample by sample. The result of thesubtraction of the pacing artifact template from the captured responsetemplate may be stored 794 as an evoked response template. The evokedresponse template may be used in subsequent cardiac responseclassification procedures.

In another embodiment, the pacing artifact template may be normalizedand canceled from a number of captured response beats. The pacingartifact template canceled beats may then be averaged to produce theevoked response template.

FIG. 7E illustrates a method of generating a pacing artifact templateaccording to embodiments of the invention. In the exemplary embodimentillustrated by FIG. 7E, a number of pace pulses are delivered 768 togenerate pacing artifact waveforms. The pace pulses are delivered insuch a way that capture does not occur. The resultant cardiac signal mayrepresent a relatively pure pacing artifact waveform without asuperimposed evoked response. Pacing artifact signals without anassociated evoked response may be produced by delivering 769 pace pulsesat an energy level lower than the pacing threshold. Alternatively, thepace pulses may be delivered 770 during a myocardial refractory period.The myocardial refractory period represents a time when the heart tissueis recovering from a previous cardiac beat. A pace pulse deliveredduring the myocardial refractory period typically does not produce anevoked response in the heart tissue, thus a pacing artifact waveform maybe acquired.

Following delivery 768 of a pace pulse using either of the above methodsdescribed in connection with blocks 769 or 770, a pacing artifactwaveform is sensed 771. The pacing artifact waveform may be averagedwith previously acquired pacing artifact waveforms 772, if any. Theprocess of generating a pace pulse and detecting the resultant pacingartifact waveform 768-772 may be repeated until a predetermined numberof pacing artifact waveforms has been acquired 773. When a sufficientnumber of pacing artifact waveforms has been acquired 773, the averagepacing artifact waveform is stored 774 as the pacing artifact template.

The pacing artifact may exhibit small variations in morphology withrespect to pace pulse amplitude. Accordingly, the use of multiple pacingartifact templates corresponding to various pace pulse amplitudes mayprovide a more thorough cancellation of the pacing artifact over a rangeof pace pulse amplitudes, e.g., as used in a pacing threshold test. Themethod illustrated in FIG. 7E may be applied to generate pacing artifacttemplates for each pacing pulse amplitude of interest.

Alternatively, or additionally, a set of two or more pacing artifacttemplates may be generated, wherein a particular pacing artifacttemplate characterizes the pacing artifact associated with a small rangeof pace pulse amplitudes. A pacing artifact template for a pace pulserange can be formed by combining pacing artifact waveforms from variouspace pulse amplitudes within the range using, for example, an averagingoperation. The pacing artifact template for a pace pulse range may alsobe formed by selecting a pacing artifact waveform at a single pace pulseamplitude, e.g., a pacing artifact waveform for a pulse amplitude nearthe center of the range to be characterized. The set of pacing artifacttemplates correspond to the entire pace pulse amplitude range to beevaluated.

The artifact waveform measurement may be accomplished during therefractory period of the myocardium. Pace pulses delivered during therefractory period produce pacing artifact waveforms without the evokedresponse components. The timing of the pace pulse delivered for pacingartifact measurement in the myocardial refractory period should beselected to be before the vulnerable period of the myocardium to avoidpro-arrhythmia, and after the deflections from the myocardial responsefrom the previous cardiac event in the chamber have passed, e.g., 80 msafter the preceding cardiac event.

Processes for CR template formation, ER template formation, and pacingartifact template formation are described in commonly owned U.S. Pat.No. 7,191,004 and U.S. Pat. No. 7,162,301, both of which areincorporated herein by reference.

The flowchart of FIG. 8A illustrates a method of cardiac responseclassification utilizing a captured response (CR) template to definemultiple classification windows according to embodiments of theinvention. A captured response template is provided 810, for example,using a technique such as the one described above. The timing of aselected characteristic of the CR template is determined 812 relative toa pacing stimulation. A pacing pulse is delivered 814 and cardiacresponse classification windows are established 816 based on the timingof the selected CR template characteristic, as illustrated in FIG. 9. Acardiac signal following the pacing pulse is sensed 818. A first cardiacsignal characteristic is detected 820 in the first classification windowand a second cardiac signal characteristic may be detected 822 in thesecond classification window.

The first characteristic is compared 824 to a first reference. If thefirst characteristic is consistent with the first reference, then thecardiac response is classified 826 as a first type of response. If thefirst characteristic is inconsistent with the reference, then the secondcharacteristic may be checked.

The second characteristic is compared to a second reference 828. If thesecond characteristic is inconsistent with the second reference, thecardiac signal is classified 830 as a second type of response. If thesecond characteristic is consistent with the second reference, then thecardiac signal is classified as a third type of response 832.

This example is further illustrated by the graph of FIG. 9. A pluralityof classification windows 950, 960, 970 are established relative to apacing pulse 910 based on the timing of a selected characteristic of theCR template. For example, the selected characteristic may comprise anextrema point, slope, curvature or other morphological featurecharacteristic of the CR template. A cardiac signal 940 following thepacing pulse is illustrated with respect to three establishedclassification windows 950, 960, 970. A first characteristic 980, inthis example, a positive peak, is detected in the first classificationwindow 960. A second characteristic, e.g., negative peak 990 is detectedin the second classification window 950. The first and the secondcharacteristics 980, 990 may be compared to references and the cardiacresponse classified as described in connection with the flowchart ofFIG. 8A.

The flowchart of FIG. 8B illustrates a method of cardiac responseclassification using an evoked response template to define multipleclassification windows in accordance with embodiments of the invention.An evoked response (ER) template and a pacing artifact template areprovided 840, for example, using the techniques described above. Thetiming of a selected characteristic of the ER template is determined 842relative to a pacing stimulation. A pacing pulse is delivered 844 andfirst and second classification windows are established 846 based on thetiming of the selected ER template characteristic. A cardiac signalfollowing the pacing pulse is sensed 848. The pacing artifact templateis subtracted from the sensed cardiac signal 850. Using the pacingartifact cancelled cardiac signal, a first cardiac signal characteristicis detected 852 in the first classification window and a second cardiacsignal characteristic is detected 854 in the second classificationwindow.

The first characteristic is compared 856 to a first reference. If thefirst characteristic is consistent with the first reference, then thecardiac response is classified 860 as a first type of response. If thefirst characteristic is inconsistent with the reference 856, then thesecond characteristic is checked 862.

The second characteristic is 862 compared to a second reference. If thesecond characteristic is inconsistent 862 with the second reference, thecardiac signal is classified 864 as a second type of response. If thesecond characteristic is consistent 862 with the second reference, thenthe cardiac signal is classified as a third type of response 866.

The cardiac response classification processes described herein may beimplemented in an autocapture process wherein capture of the heart isverified on a beat-by-beat basis during pacing. If a pacing stimulationdoes not produce a captured response, a variety of interventions may beeffected, including, for example, delivering a back-up pacingstimulation at a higher energy level and/or initiating a capturethreshold test to determine the capture threshold of the cardiac tissue.In accordance with embodiments of the invention, a method for performingcardiac response classification that is particularly suitable forimplementation in an autocapture process is illustrated in the flowchartof FIG. 10. Although the process described in FIG. 10 is described inconnection with an autocapture procedure, the process may beadvantageously applied in other procedures in connection with cardiacresponse classification.

Classification windows are defined 1010 based on the timing of the peakof the CR template. A cardiac signal following a pacing stimulation issensed 1020. The peak of the sensed cardiac signal is detected 1030 inone of the classification windows. Classification of the cardiacresponse is performed 1040 based on the amplitude of the peak and theparticular classification window in which the peak is sensed.

FIG. 11 is a graph illustrating the implementation of classificationwindows that may be established in connection with the method of FIG.10. In this example, three classification windows 1130, 1140, 1150 areestablished based on the timing of the peak of the CR template 1120relative to the timing of the delivery of the pacing stimulation 1110.In this example, a first classification window 1130 is associated with afusion/pseudofusion response, the second window 1140 is associated witha captured response, and the third window 1150 is associated withintrinsic beats. The second classification window 1140 may be centeredabout the timing of the CR template peak 1120 and includes predeterminedintervals before 1125 and after 1126 the CR template peak 1120, e.g.,intervals of about 10 ms.

The flowchart of FIG. 12 illustrates a method of classifying a cardiacresponse using the classification windows described in FIG. 11. A CRtemplate is generated 1210 and the timing of the peak of the CR templateis determined. A pacing stimulation is delivered 1215. Classificationwindows, including a fusion/pseudofusion window, a captured responsewindow and a non-captured response window, are established 1220 based onthe timing of the CR template peak relative to the pacing stimulation.The peak of a cardiac signal sensed following the pacing stimulation isdetected 1225 in one of the classification windows.

If the cardiac signal is determined to be noisy 1226, then cardiacresponse classification is not performed for the pacing stimulation andthe process continues. Commonly owned U.S. Pat. No. 6,505,071, which isincorporated herein by reference, describes methods and systems that maybe utilized for noise detection in the context of the cardiac responseclassification processes in accordance with embodiments of theinvention. If noise is not detected 1226 and if the amplitude of thedetected peak is less than 1230 a reference value, then the cardiacresponse is classified 1235 as a non-captured response. In oneembodiment, reference values used in connection with cardiac responseclassification may be dynamic references that are adjusted based onrespiration, activity level, and lead maturation, among other factors asdescribed in commonly owned U.S. Pat. No. 6,192,275 which isincorporated herein by reference. In another embodiment, the referencevalue may be a predetermined percentage, e.g., 50% of the capturedresponse template peak. The cardiac response may be classified as anon-captured response if the cardiac signal exhibits a peak that is lessthan 50% of the captured response template peak, where the cardiacsignal peak and the CR template peak have the same sign.

If the peak of the sensed cardiac signal is detected 1240 in thefusion/pseudofusion classification window, the cardiac response isclassified 1245 as fusion or pseudofusion. If the peak of the sensedcardiac signal is detected 1250 in the captured response classificationwindow, the cardiac response is classified 1255 as a captured response.If the peak of the cardiac signal is not detected in thefusion/pseudofusion window or the capture window, it is detected 1260 inthe intrinsic classification window, and the cardiac signal isclassified 1265 as an intrinsic beat.

FIGS. 13-15 illustrate a method of using the peak width of a cardiacsignal sensed after a pacing stimulation to classify a cardiac responsein accordance with embodiments of the invention. Such a method may beparticularly useful, for example, in an automatic capture detectionprocess performed on a beat-by-beat basis, but may also be used inconnection with other procedures involving the classification of cardiacresponses to pacing. According to the process illustrated in FIGS.13-15, the peak width of the sensed cardiac signal is compared to one ormore peak width references to determine the cardiac response. FIG. 13illustrates the classification windows 1305, 1310, 1315 used to classifythe cardiac response. The peak width of the cardiac signal may beestablished as a time interval that the cardiac signal remains above apredetermined percentage, e.g., 10%, or other amount, of the cardiacsignal peak amplitude.

The classification windows 1305, 1310, 1315 may be established, forexample, based on the time of the peak of a CR template 1302 relative tothe time of the pacing stimulation 1301. In one example, a firstclassification window 1310 may be associated with a captured response, asecond classification window 1305 may be associated with afusion/pseudofusion response, and a third classification window 1315 maybe associated with an intrinsic beat. If the peak of the sensed cardiacsignal exceeds a predetermined amplitude, e.g., about 50% of the CRtemplate peak amplitude, and is detected in one of the classificationwindows 1305, 1310, 1315, the peak width of the detected cardiac signalis compared to one or more peak width references. The peak widthreferences may comprise a single peak width value, or a range of values,for example. FIG. 13 illustrates four peak width references 1320, 1330,1350, 1360.

In this example, the each of the peak width references is associatedwith a range of peak widths. The peak width references are determinedbased on the peak width of the CR template, the peak width associatedwith a template characterizing an intrinsic cardiac beat (I template),or both. The CR template may be established as previously described. Thepeak width of the CR template may be established as the time intervalthat the CR template waveform remains above a predetermined percentage,e.g., 10%, or other amount, of the CR template waveform peak amplitude.

An intrinsic template (I template) characterizes the patient'ssupraventricular conducted rhythm (SVR). The I template may be formedfrom a combination of one or more beats, wherein the each beatrepresents the patient's SVR rhythm. The peak width of the I templatemay be determined, for example, as an average of the peak widths of theone or more beats used to form the I template. The peak width of each ofthe one or more beats used to form the I template may be established asthe time interval that the beat waveform remains above a predeterminedpercentage, e.g., 10%, or other amount, of the peak amplitude. Specificembodiments involving I template formation are described in more detailbelow with reference to FIGS. 19-24.

FIGS. 14A and 14B illustrate the peak widths of cardiac signalsrepresentative of a captured response 1410 and an intrinsic beat 1450,respectively. The peak width of the captured response 1420 may beestablished, for example, as the time period that the cardiac signalremains above a predetermined percentage, for example, 10% or otheramount, of the captured response peak. Similarly, the peak width of theintrinsic beat 1460 may be established as the time period that thecardiac signal remains above a predetermined percentage of the intrinsicbeat peak.

FIG. 13 illustrates a first peak width reference 1320 comprising therange of peak widths less than or equal to the I template peak widthminus B, a first predetermined amount, e.g., 10 ms or other value. Asecond peak width reference 1330 comprises a range of peak widthsgreater than the I template peak width minus B and less than or equal toan average of the CR template and I template peak widths. A third peakwidth reference 1350 may be established as the range of peak widthsexceeding the average of the CR template and I template peak widths andless than or equal to the CR template peak width plus A, a secondpredetermined amount, e.g., 10 ms or other value. A fourth peak widthreference 1360 may be established as the range of peak widths exceedingthe CR template peak width plus A. The values of A and B may bedetermined based on the morphologies of the CR template and/or theintrinsic template, for example. In one implementation, the values of Aand/or B may be determined based on the peak widths of the CR templateand/or the intrinsic template. Further, the values of A and/or B may beadapted to track slow changes in the CR template and/or intrinsictemplate morphology.

The values of A and/or B may be set adaptively in response to thestatistics of the variability of the peak widths over sample sets ofcaptured and intrinsic beats. For example, A and B may be set to includea predetermined number of standard deviations of the peak width, e.g., 3standard deviations.

The flowchart of FIG. 15 illustrates a method of classifying a cardiacresponse to a pacing stimulation in accordance with embodiments of theinvention. The processes of FIG. 15 use the classification windows andpeak width references established as illustrated in the graph of FIG.13.

A pacing stimulation is delivered 1505 to the heart and a cardiac signalfollowing the pacing stimulation is sensed. The amplitude and width ofthe cardiac signal peak are determined. If the cardiac signal peak hasinsufficient amplitude 1510, e.g., less than 50% of the CR templatepeak, then the cardiac response is classified 1515 as a non-capturedresponse. If the cardiac signal peak has an amplitude greater than orequal to 50% of the CR template peak and the cardiac signal peak isdetected 1520 in the second classification window, then the cardiacresponse is classified 1525 as a fusion/pseudofusion response.

If the cardiac signal peak is detected 1530 in the first classificationwindow, then the peak width of the cardiac signal is compared to one ormore peak width references to classify the cardiac response. If the peakwidth (PW) of the cardiac signal falls 1560 within the range of thethird peak width reference, (CR template peak width+I template peakwidth)/2≦PW<CR template peak width+A, then the cardiac response isclassified 1565 as a captured response. If the peak width (PW) of thecardiac signal falls 1570 within the range of with the second peak widthreference, I template peak width−B≦PW< (CR template peak width+Itemplate peak width)/2, then the cardiac response is classified 1575 asnon-captured and intrinsic. If the peak width of the cardiac signal doesnot fall 1570 within the range of either the second or third peak widthreferences, then it falls into the ranges of the first peak widthreference, PW≦CR template peak width A, or the fourth peak widthreference, PW<I template peak width−B, and is classified 1580 as noise.

If the cardiac signal peak is not detected 1520 in the secondclassification window and is also not detected 1530 in the firstclassification window, then it falls within the third classificationwindow. The cardiac signal peak width is compared to one or more peakwidth references to determine the cardiac response. If the peak widthfalls 1535 within the range of the third peak width reference, thecardiac response is classified 1540 as near non-capture. A nearnon-captured response comprises a response that occurs when the pacingstimulation is captured but delayed.

If the peak width of the cardiac signal falls 1545 within the range ofthe second peak width reference, then the cardiac response is classified1555 as a non-captured response plus an intrinsic beat. If the peakwidth of the cardiac signal does not fall 1545 within the range ofeither the second peak width reference or the third peak widthreference, then the peak width falls into either the range of the firstpeak width reference or the fourth peak width reference and isclassified 1550 as noise.

The cardiac response classification methods of the invention asdescribed below may be particularly useful in an automatic capturethreshold determination procedure. A capture threshold test mayinitially deliver pacing at a high energy level, thus ensuring capturedresponses. The pacing energy level may be ramped down from the initialhigh energy level until loss of capture is detected. The point justbefore loss of capture occurs may be established as the capturethreshold.

The flowchart of FIG. 16 illustrates a process that is particularlyuseful for performing a capture threshold determination process based oncardiac response classification in accordance with embodiments of theinvention. Although this process is described in terms of a capturethreshold procedure, the methods of cardiac response classification maybe used in connection with other procedures including, for example, beatby beat automatic capture verification.

A sequence of pacing pulses are delivered 1610 to the heart. Forexample, the pacing pulses may have an initially high energy level withthe pacing energy level decreasing in discrete steps. For each deliveredpace pulse, a plurality of classification windows are established 1620relative to and following the time of each pace pulse. Cardiac signalsfollowing the pacing pulses are sensed 1630 within the classificationwindows. The cardiac signals are compared 1640 to referencesrespectively associated with different types of cardiac responses. Thecardiac response to each of the pace pulses is classified 1650 based onthe comparisons. The classifications of the cardiac responses are used1660 to determine a pacing energy capture threshold.

In one example, the pacing energy of the pacing pulses is ramped downfrom an initially high pacing energy. The cardiac response following thedelivery of each pacing pulse is determined as described in connectionwith the flowchart of FIG. 16. When a predetermined number of pacingpulses produce a non-captured response, e.g., about two out of threedelivered pacing pulses, loss of capture is determined. The point justbefore loss of capture occurs comprises the capture threshold.

FIG. 17 is a diagram illustrating classification windows and referencesthat may be used in determining a cardiac pacing response according toembodiments of the invention. In this example embodiment, first, second,and third classification windows 1730, 1740, 1750 are established basedon the timing of a particular CR template characteristic 1720, such asthe peak of the CR template, relative to the time of the delivery of thepace pulse 1710. A selected characteristic of the cardiac signalfollowing the pace pulse, e.g., the peak of the cardiac signal, isdetected in one of the classification windows 1730, 1740, 1750.Depending upon the particular classification window in which theselected characteristic is detected, features of the cardiac signal arecompared to one or more references. The references may include, forexample, templates characterizing various types of cardiac responses,including the CR template and/or the I template. The cardiac response isclassified based on the comparison of the cardiac signal feature orfeatures and the one or more references, and the particular window inwhich the selected characteristic of the cardiac signal is detected.

The flowchart of FIG. 18 provides a more detailed illustration of amethod of classifying a cardiac response following a pacing pulse basedon a comparison between the cardiac signal and the CR template or the Itemplate, for example. The method described with reference to FIGS. 17and 18 make use of cardiac signal morphology templates including thecaptured response (CR) template and the intrinsic response I template.The method described in connection with FIG. 18 may be particularlyuseful in connection with an automatic capture threshold detectionprocedure.

As discussed previously, a captured response (CR) template exemplifies awaveform representative of a captured response. The CR template may bederived from a waveform that is produced when a pacing pulse capturesthe heart, and includes both the evoked response and the superimposedpacing artifact. A CR template may comprise, for example, a sequence ofsamples or feature points of a cardiac signal representing a capturedresponse.

An intrinsic response template, referred to herein as an I template,characterizes the morphology of an electrical signal associated with thepatient's intrinsic or supraventricular conducted cardiac rhythm (SVR).Processes for forming templates representing the patient'ssupraventricular conducted rhythm (SVR) using a two channel procedureare described in commonly owned U.S. Pat. Nos. 6,708,058; 7,184,818;7,085,599; 6,889,079; and 6,449,503 all of which are incorporated hereinby reference.

An I template may be formed from a combination of one or more beats,wherein each beat represents the patient's intrinsic or SVR rhythm.Cardiac beats used to form the I template may be required to meetcertain criteria, such as stability and/or rate criteria. According toone embodiment, an I template generation process involves sensingcardiac signals on a rate channel and on a shock channel. Shock channelautomatic gain control may be performed prior to collecting beats for Itemplate generation. For example, the shock channel gain control may beeffected by measuring the peak value in four beats meeting certain rateand stability criteria and adjusting the shock channel gain such thatthe averaged peak value is 50% of the maximum A/D converter value.

According to a two channel approach for template generation, a peak ofthe rate channel signal is determined and identified as the fiducialpoint. The value and location of features of the initial shock channelwaveform are determined relative to the rate channel fiducial point.Additional cardiac signals, including rate channel signals and shockchannel signals are sensed. The fiducial points for the additional ratechannel signals are determined. The shock channel waveforms are thenaligned with the I template using the fiducial points developed from therate channel signals. The I template is generated using featuresextracted from the aligned shock channel waveforms.

A fiducial point represents a peak value of the rate channel signal. Afiducial point type is either positive (Pos), associated with a positivepeak, or negative (Neg), associated with a negative peak. When atemplate is formed, the positive peak (Pos) or the negative peak (Neg)of the rate channel signal used to form the template determines thefiducial point type of the template. FIGS. 19 and 20 depict positive andnegative fiducial points, respectively. The Pos and Neg peaks aremeasured as absolute values. The fiducial point type is determined byEquation 3 as follows:If Pos>0.9*Neg, the fiducial point type is positiveIf Pos≦0.9*Neg, the fiducial point type is negative  [3]

If a stored I template exists, the fiducial point type of the storedtemplate is used as the fiducial point type of the template. If nostored template exists, the fiducial point type of the first beat usedto form the template is used as the fiducial point type for thetemplate.

In one embodiment of the invention, and with reference to FIGS. 21 and22, five features are initially identified for the I template, followedby three additional features determined at midpoints between certainones of the five initially selected features.

Feature 3 is selected as the absolute maximum peak in a feature windowdefined by 31 samples centered at the fiducial point. If the positivepeak amplitude is equal to the negative peak amplitude, the positivepeak is selected as Feature 3.

Feature 2 is found by searching backward from Feature 3 until a point isreached that meets the following conditions: 1) the search is limited to10 samples. If no point satisfies the following conditions, then the10th sample becomes Feature 2; 2) the amplitude is less than 25% of themaximum peak; 3) a turning point is found or the slope is flat, and 4)Feature 2 is at least 4 samples away from Feature 3.

By way of example, let Q(I) represent the current sample. A turningpoint is found if:Q(I−1)≧Q(I) and Q(I)<Q(I+1) for a positive FeatureQ(I−1)≧Q(I) and Q(I)>Q(I+1) for a negative Feature 3  3[4]

As is shown in FIG. 21, Q(I) is selected as Feature 2. As such, Feature2 is selected as a turning point.

The slope is considered flat, as shown in FIG. 22, ifabs(Q(I+1)−Q(I−1))<4 and abs(Q(I+1)−Q(I−2))<4, in the case when the A/Dconverter maximum value is 128. In the illustrative depiction of FIG.22, Q(I) is selected as Feature 2. As such, Feature 2 is selected as aflat slope point.

Feature 4 is found by searching forward starting from Feature 3 until apoint is reached that meets the following conditions: 1) the search islimited to 16 samples. If no point satisfies the following conditions,then the 16th sample becomes Feature 4; 2) the amplitude is less than25% of the maximum peak; and 3) a turning point is found or the slope isflat.

By way of example, let Q(I) represent the current sample. A turningpoint is found if:Q(I+1)≧Q(I) and Q(I)<Q(I−1) for a positive Feature 3Q(I+1)≦Q(I) and Q(I)>Q(I−1) for a negative Feature 3  [5]

Q(I) is selected as Feature 4, as is shown in FIG. 23.

The slope is flat, as shown in FIG. 24, if abs(Q(I−1)−Q(I+1))<4 andabs(Q(I-1)−Q(I+2))<4. In this case, Q(I) is selected as Feature 4.

Feature 1 is selected as the seventeenth sample from the beginning ofthe detection window. Feature 5 is selected as the last sample of thedetection window. Three additional features are selected at the midpointof Features 1 and 2, the midpoint of Features 2 and 3, and the midpointof Features 3 and 4, respectively. If a midpoint falls between twosample points, the leftmost (earlier in time) point is selected. Thus,according to this embodiment, eight feature values (e.g., amplitudes)and their associated locations with respect to the fiducial point andthe corresponding fiducial point type are saved as the I template.

Following generation of an I template, a subsequently detected cardiacbeat may be compared to the I template to classify the cardiac beat. Ifthe cardiac beat has a morphology similar to that of an intrinsic beat,then the features of the cardiac beat will be correlated to the templatefeatures. Various steps associated with determining if a cardiac beat iscorrelated to an I template in accordance with embodiments of theinvention are described below.

The rate channel signal and the shock channel signal for the cardiacbeat are sensed. The fiducial point of the rate channel signal isdetermined. The rate channel fiducial point is used to align the rateand shock channel waveforms of the cardiac beat with the template.Features of the shock channel signal are determined at the locationsrelative to the fiducial point previously determined for the template.The template features and the cardiac signal features are compared bycalculating a feature correlation coefficient (FCC). In one particularembodiment, Equation 6, provided below, is used to compute the FCCbetween the template features and the cardiac signal features.

$\begin{matrix}{{FCC} = \frac{\left( {{N{\sum\limits_{i = 1}^{N}\;{X_{i}Y_{i}}}} - {\left( {\sum\limits_{i = 1}^{N}\; X_{i}} \right)\left( {\sum\limits_{i = 1}^{N}\; Y_{i}} \right)}} \right)^{2}}{\left( {{N{\sum\limits_{i = 1}^{N}\; X_{i}^{2}}} - \left( {\sum\limits_{i = 1}^{N}\; X_{i}} \right)^{2}} \right)\left( {{N{\sum\limits_{i = 1}^{N}\; Y_{i}^{2}}} - \left( {\sum\limits_{i = 1}^{N}\; Y_{i}} \right)^{2}} \right)}} & \lbrack 6\rbrack\end{matrix}$where, Xi represents template N features and Yi represents beat Nfeatures, and N=8 in this illustrative example. The sign of thenumerator term is checked before squaring. If the numerator is negative,the beat is uncorrelated, and the remainder of the computation need notbe performed.

If the FCC is greater than a predetermined value, for example 0.94, thenthe cardiac beat is correlated to the template. If the FCC is less thanor equal to the predetermined value, then the cardiac beat isuncorrelated to the template.

Alternatively, correlation between the cardiac beat and the template maybe calculated using the CWA technique described by Equation 1 above.Other techniques may also be implemented to generate templates and tocompare templates and the cardiac signals.

For example, an alternate methodology for generating templatesrepresentative of various types of cardiac signals is described incommonly owned U.S. Pat No. 6,684,100, which is incorporated herein byreference. The application cited immediately above describes acurvature-based method for selecting features of a template, e.g., an Itemplate for example. The curvature-based method of template formationmay be used in the cardiac response classification processes describedherein.

The flowchart of FIG. 18 illustrates a method for classifying a cardiacsignal following deliver of a pacing pulse based on comparison of thecardiac signal one or more of a CR template and an I template. Followingthe delivery 1810 of a pace pulse to the heart, classification windowssuch as those described in connection with FIG. 17 are establishedrelative to the timing of the pace pulse. The peak of the cardiac signalfollowing the pace pulse is detected in one of the establishedclassification windows. The peak amplitude of the cardiac signal isdetermined.

If noise is detected 1812, then cardiac response classification is notperformed for the pacing stimulation and the process continues. If noiseis not detected 1812 and if the peak amplitude of the cardiac signal isless than 1815 a predetermined value, for example, about 50% of the CRtemplate peak amplitude, then the cardiac response is classified as anon-captured response. If the peak amplitude of the cardiac signal isgreater than or equal to 1815 the predetermined value, then the cardiacsignal may be compared to one or more references to classify the cardiacresponse to the pacing stimulation.

If the cardiac signal peak amplitude is greater than or equal to 1815the predetermined value, then cardiac signal is compared to theintrinsic template. If the cardiac signal is correlated 1825 to the Itemplate then the cardiac response is classified 1865 as a non-capturedresponse and intrinsic beat. Correlation may be determined bycalculating a feature correlation coefficient representing the degree ofcorrelation between the cardiac signal and the I template using Equation5 above. For the purposes of cardiac response classification, a cardiacsignal is determined to be correlated to the intrinsic beat template ifthe FCC is about 0.94.

If the cardiac signal is not correlated 1825 to the I template, thencorrelation with the CR template is checked 1850. The comparison of thecardiac signal to the CR template may be performed, for example, bycalculating a correlation coefficient (CC) representing the degree ofcorrelation between the cardiac signal and the captured responsetemplate using a technique such as Correlation Waveform Analysis (CWA).In one particular embodiment, Equation 1, provided above, is used tocompute the CC between the samples of a cardiac signal and the capturedresponse template samples. Typically, the number of samples used for thecalculation is about 33 samples. If the correlation coefficient isgreater than a predetermined value, for example, about 0.94, the cardiacsignal is considered to be correlated to the CR template. If the cardiacsignal is not correlated 1850 to the CR template, then the cardiacresponse is classified 1860 as a fusion/pseudofusion response.

If the cardiac signal is correlated 1850 to the CR template and the peakof the cardiac signal is detected 1830 within the third classificationwindow, illustrated in FIG. 17, then the cardiac response is determined1835 to be near non capture. If the peak of the cardiac signal is notdetected 1830 in the third classification window, then the cardiacresponse is classified 1840 as capture and the CR template may beupdated 1845 using the cardiac signal.

Although the examples illustrated in FIGS. 11-18 are described in termsof using a CR template for cardiac response classification, similarapproaches may be implemented using an ER template. In processes usingan ER template, the pacing artifact template may be subtracted orotherwise cancelled from the sensed cardiac signal prior to analyzingthe sensed cardiac signal to determine the cardiac response.

Various modifications and additions can be made to the preferredembodiments discussed hereinabove without departing from the scope ofthe present invention. Accordingly, the scope of the present inventionshould not be limited by the particular embodiments described above, butshould be defined only by the claims set forth below and equivalentsthereof.

1. A method of classifying a cardiac response to a pacing pulse,comprising: defining a plurality of classification windows relative toand subsequent to the pacing pulse wherein the plurality of windowscomprise, a first classification window predominantly associated with acaptured response, a second classification window predominantlyassociated with a fusion/pseudofusion response, and a thirdclassification window predominantly associated with a non-capturedresponse; sensing a cardiac signal following the pacing pulse; detectingone or more characteristics of the cardiac signal within one or moreparticular classification windows of the plurality of classificationwindows; and classifying the cardiac response to the pacing pulse basedon the one or more detected characteristics and the one or moreparticular classification windows.
 2. The method of claim 1, furthercomprising: detecting noise on the cardiac signal; and canceling theclassification of the cardiac response based on the detection of noise.3. The method of claim 1, wherein defining the plurality ofclassification windows comprises defining a plurality of non-overlappingclassification windows.
 4. The method of claim 1, wherein defining theplurality of classification windows comprises adapting a timing of aparticular classification window.
 5. The method of claim 1, whereindefining the plurality of classification windows comprises adapting aduration of a particular classification window.
 6. The method of claim1, wherein defining the plurality of classification windows comprises:defining a first classification window based on a timing of a cardiacsignal feature associated with a captured response; and defining one ormore additional classification windows in relation to the firstclassification window.
 7. The method of claim 6, wherein defining thefirst classification window based on the timing of the featureassociated with the captured response comprises defining the firstclassification window based on the timing of a captured responsetemplate peak.
 8. The method of claim 6, wherein defining the firstclassification window based on the timing of the feature associated withthe captured response comprises defining the first classification windowbased on the timing of an evoked response template peak.
 9. The methodof claim 6, wherein defining the one or more additional classificationwindows comprises: defining a second classification window to occurbefore the first classification window; and defining a thirdclassification window to occur after the first classification window.10. The method of claim 1, wherein detecting the one or morecharacteristics of the cardiac signal comprises detecting one or moremorphological features of the cardiac signal.
 11. The method of claim 1,wherein detecting the one or more characteristics of the cardiac signalcomprises detecting a peak of the cardiac signal.
 12. The method ofclaim 1, wherein detecting the one or more characteristics of thecardiac signal comprises detecting a peak width of the cardiac signal.13. The method of claim 1, wherein classifying the cardiac responsecomprises comparing the one or more characteristics to one or morereferences respectively associated with one or more types of cardiacresponses.
 14. The method of claim 13, wherein the one or morereferences are adaptable.
 15. The method of claim 13, wherein aparticular reference is adaptable based on a statistical variability ofthe particular reference.
 16. The method of claim 1, wherein classifyingthe cardiac response comprises classifying the cardiac response as acaptured response.
 17. The method of claim 1, wherein classifying thecardiac response comprises classifying the cardiac response as anon-captured response.
 18. The method of claim 1, wherein classifyingthe cardiac response comprises classifying the cardiac response as anon-captured response and an intrinsic beat.
 19. The method of claim 1,wherein classifying the cardiac response comprises classifying thecardiac response as a near non-captured response.
 20. The method ofclaim 1, wherein classifying the cardiac response comprises classifyingthe cardiac response as a fusion/pseudofusion beat.
 21. The method ofclaim 1, wherein: detecting the one or more characteristics of thecardiac signal comprises detecting an amplitude of the cardiac signal;and classifying the cardiac response comprises: comparing the detectedamplitude to an amplitude reference; and classifying the cardiacresponse based on the comparison.
 22. The method of claim 1, wherein:detecting the one or more characteristics of the cardiac signalcomprises detecting a slope of the cardiac signal; and classifying thecardiac response comprises: comparing the detected slope to a slopereference; and classifying the cardiac response based on the comparison.23. The method of claim 1, wherein: detecting the one or morecharacteristics of the cardiac signal comprises detecting a curvature ofthe cardiac signal; and classifying the cardiac response comprises:comparing the detected curvature to a curvature reference; andclassifying the cardiac response based on the comparison.
 24. The methodof claim 1, wherein: detecting the one or more characteristics of thecardiac signal comprises detecting a peak width of the cardiac signal;and classifying the cardiac response comprises: comparing the detectedpeak width to a peak width reference; and classifying the cardiacresponse based on the comparison.
 25. The method of claim 1, wherein:detecting the one or more characteristics of the cardiac signalcomprises detecting one or more feature points of the cardiac signal;and classifying the cardiac response comprises: providing a template;comparing the one or more feature points to the template; andclassifying the cardiac response based on the comparison.
 26. The methodof claim 25, wherein providing the template comprises providing acaptured response template.
 27. The method of claim 26, whereinproviding the captured response template comprises updating the capturedresponse template using one or more additional captured response beats.28. The method of claim 26, wherein providing the captured responsetemplate comprises forming the captured response template using one ormore captured response beats.
 29. The method of claim 26, whereinforming the captured response template comprises: delivering one or morepacing pulses at a energy level exceeding a capture threshold; sensingone or more cardiac signals following the pacing pulses; and forming thecaptured response template using the one or more cardiac signals. 30.The method of claim 29, wherein delivering the one or more pacing pulsesat an energy level exceeding the capture threshold comprises deliveringthe one or more pacing pulses during a capture threshold test.
 31. Themethod of claim 25, wherein providing the template comprises providingan intrinsic beat template.
 32. The method of claim 31, whereinproviding the intrinsic beat template comprises forming the intrinsictemplate using one or more cardiac beats representative of an intrinsiccardiac beat.
 33. The method of claim 32, wherein forming the intrinsicbeat template using one or more beats representative of an intrinsiccardiac beat comprises: sensing rate channel signals of the one or morecardiac beats; sensing shock channel signals of the one or more cardiacbeats; and forming the intrinsic template using the rate channel signalsand the shock channel signals.
 34. The method of claim 25, whereinproviding the template comprises providing an evoked response template.35. The method of claim 34, wherein providing the evoked responsetemplate comprises: providing a captured response template; canceling apacing artifact from the captured response template; and forming theevoked response template using the pacing artifact canceled capturedresponse template.
 36. A method of classifying cardiac pacing responses,comprising: defining a plurality of classification windows relative andsubsequent to each pacing stimulation; sensing cardiac signals followingthe pacing stimulations; detecting characteristics of the cardiacsignals within particular classification windows, comparing one or morecharacteristics of a cardiac signal to a captured response template andan intrinsic response template; and classifying a cardiac pacingresponse as a fusion/pseudofusion beat if the one or morecharacteristics are not consistent with the captured response templateor the intrinsic response template.
 37. The method of claim 36, furthercomprising: detecting noise on the cardiac signals; and cancelingclassification of the cardiac pacing responses based on the detection ofnoise.
 38. The method of claim 36, further comprising using theclassifications of the cardiac pacing responses to determine a pacingenergy capture threshold.
 39. The method of claim 38, wherein;delivering the sequence of pacing stimulations to a heart comprisesdelivering a sequence of pacing stimulations of variable energy; andusing the classifications to determine the pacing energy capturethreshold comprises: detecting loss of capture if a predetermined numberof cardiac pacing responses are classified as non-captured responses;and determining the pacing energy capture threshold based on the loss ofcapture detection.
 40. The method of claim 39, wherein detecting loss ofcapture comprises detecting loss of capture if a predetermined number ofcardiac responses are classified as non-captured responses.
 41. Themethod of claim 40, wherein the predetermined number of cardiacresponses is about two out of about three cardiac responses.
 42. Themethod of claim 41, further comprising generating a captured responsetemplate using one or more cardiac signals associated with capturedresponses.
 43. The method of claim 39, wherein delivering the sequenceof pacing stimulations comprises delivering a sequence of pacingstimulations of decreasing energy.
 44. The method of claim 39, whereindelivering the sequence of pacing stimulations comprises delivering asequence of pacing stimulations of increasing energy.
 45. The method ofclaim 36, wherein classifying the cardiac pacing responses comprisesclassifying a non-captured response.
 46. The method of claim 36, whereinclassifying the cardiac pacing responses comprises classifying acaptured response.
 47. The method of claim 36, wherein classifying thecardiac pacing responses comprises classifying a fusion/Pseudofusionresponse.
 48. The method of claim 36, wherein classifying the cardiacpacing responses comprises classifying a near non-captured response. 49.The method of claim 36, wherein classifying the cardiac pacing responsescomprises classifying a non-captured response and an intrinsic beat. 50.The method of claim 36, wherein: comparing the characteristics to theone or more references comprises comparing a peak amplitude of aparticular cardiac signal to a peak amplitude reference; and classifyingthe cardiac pacing responses comprises classifying a particular cardiacresponse as a non-captured response if the particular cardiac signal hasa peak amplitude less than the peak amplitude reference.
 51. The methodof claim 36, wherein: detecting the characteristics within theparticular classification windows comprises detecting one or morecharacteristics of a particular cardiac signal within a particularclassification window associated with a peak time of a captured responsetemplate; comparing the characteristics to the one or more referencescomprises comparing the one or more characteristics to the capturedresponse template; and classifying the cardiac pacing responsescomprises classifying a particular cardiac pacing response as a capturedresponse if the one or more characteristics are consistent with thecaptured response template.
 52. The method of claim 36, wherein:detecting the characteristics within the particular classificationwindows comprises detecting one or more characteristics of a particularcardiac signal within a particular classification window associated witha peak time of an evoked response template; comparing thecharacteristics to the one or more references comprises comparing theone or more characteristics to the evoked response template; andclassifying the cardiac pacing responses comprises classifying aparticular cardiac pacing response as a captured response if the one ormore characteristics are consistent with the evoked response template.53. The method of claim 36, wherein: comparing the characteristics tothe one or more references comprises comparing one or morecharacteristics of a particular cardiac signal to an intrinsic beattemplate; and classifying the cardiac pacing responses comprisesclassifying a particular cardiac pacing response as a non-capturedresponse and an intrinsic beat if the one or more characteristics areconsistent with the intrinsic beat template.
 54. The method of claim 36,wherein: detecting characteristics of cardiac signals following thepacing stimulations comprises: canceling a pacing artifact from eachsensed cardiac signal; and detecting one or more characteristics of eachsensed cardiac signal within one or more particular classificationwindows; comparing the one or more characteristics to one or morereferences respectively associated with types of cardiac pacingresponses; and classifying a cardiac pacing response based on thecomparisons and the one or more particular classification windows.
 55. Acapture verification method, comprising: delivering a pacingstimulation; defining a plurality of classification windows relative andsubsequent to the pacing stimulation wherein the plurality of windowscomprise, a first classification window predominantly associated with acaptured response, a second classification window predominantlyassociated with a fusion/pseudofusion response, and a thirdclassification window predominantly associated with a non-capturedresponse; sensing a cardiac signal following the pacing stimulation;detecting a peak of the sensed cardiac signal occurring within aparticular classification window; and determining a cardiac responsebased on the detected peak and the particular classification window. 56.The method of claim 55, wherein detecting the peak of the sensed cardiacsignal comprises detecting a peak having an amplitude greater than areference value.
 57. The method of claim 56, wherein the reference valueis about 50% of a captured response template peak.
 58. The method ofclaim 56, wherein the reference value is adaptable.
 59. The method ofclaim 55, wherein determining the cardiac response based on the detectedpeak and the particular classification window comprises classifying thecardiac response as a captured response if the detected peak has anamplitude greater than a reference value and occurs within the firstclassification window.
 60. The method of claim 55, wherein determiningthe cardiac response based on the detected peak and the particularclassification window comprises determining a fusion/pseudofusionresponse if the detected peak has an amplitude greater than a referencevalue and occurs within the second classification window.
 61. The methodof claim 55, wherein determining the cardiac response based on thedetected peak and the particular classification window comprisesdetermining an intrinsic response if the detected peak has an amplitudegreater than a reference value and occurs within the thirdclassification window.
 62. The method of claim 55, wherein: detectingthe peak of the sensed cardiac signal occurring within a particularclassification window comprises determining a peak width of the cardiacsignal; and determining the cardiac pacing response comprises comparingthe cardiac signal peak width to one or more peak width references. 63.The method of claim 55, wherein: detecting the peak of the sensedcardiac signal comprises determining a peak width of the cardiac signalpeak; and determining the cardiac response comprises: comparing thecardiac signal peak width to a captured response peak width reference;and classifying the cardiac response as a captured response if thecardiac signal peak width is consistent with the captured response peakwidth reference.
 64. The method of claim 55, wherein: detecting the peakof the sensed cardiac signal comprises determining the peak width of thecardiac signal peak; and determining the cardiac response comprises:comparing the cardiac signal peak width to a non-captured response peakwidth reference; and classifying the cardiac response as a non-capturedresponse if the cardiac signal peak width is consistent with thenon-captured response peak width reference.
 65. The method of claim 55,wherein: detecting the peak of the sensed cardiac signal comprisesdetermining the peak width of the cardiac signal peak; and determiningthe cardiac response comprises: comparing the cardiac signal peak widthto a noise peak width reference; and classifying the cardiac response asnoise if the cardiac signal peak width is consistent with the noise peakwidth reference.
 66. The method of claim 55, wherein: detecting the peakof the sensed cardiac signal comprises determining a width of thecardiac signal peak; and determining the cardiac response comprises;comparing the cardiac signal peak width to a near non-captured responsepeak width reference; and classifying the cardiac response as a nearnon-captured response if the cardiac signal peak width is consistentwith the near non-captured response peak width reference.
 67. The methodof claim 55, wherein: detecting the peak of the sensed cardiac signalcomprises determining a peak width of the cardiac signal peak; anddetermining the cardiac response comprises: comparing the cardiac signalpeak width to a non-captured response and intrinsic beat peak widthreference; and classifying the cardiac response as a non-capturedresponse and an intrinsic beat if the cardiac signal peak width iswithin the non-captured response and intrinsic beat peak widthreference.
 68. A medical device, comprising: a pacing pulse deliverycircuit configured to deliver a pacing pulse to a heart; a sensingcircuit configured to sense a cardiac signal associated with the pacingpulse; and a control circuit, coupled to the sensing circuit andconfigured to define a plurality of classification windows relative toand following the pacing pulse, wherein the plurality of windowscomprise a first classification window predominantly associated with acaptured response, a second classification window predominantlyassociated with a fusion/pseudofusion response, and a thirdclassification window predominantly associated with a non-capturedresponse, detect one or more characteristics of the cardiac signalsensed within one or more particular classification windows, andclassify a cardiac response to the pacing pulse based on the one or morecharacteristics and the particular classification windows.
 69. Thedevice of claim 68, wherein the sensing circuit is configured to sensethe cardiac signal through a sensing electrode combination that is lesssensitive to a pacing artifact relative to a pacing electrodecombination.
 70. The device of claim 68, wherein the sensing circuit isconfigured to sense the cardiac signal through an electrode combinationassociated with a signal propagation delay relative to a pacing pulsedelivery time.
 71. The device of claim 68, wherein: the pacing pulsedelivery circuit is configured to deliver pacing stimulations usingelectrodes associated with a near-field vector; and the sensing circuitis configured to sense the cardiac signals using a far-field vector. 72.The device of claim 68, wherein the control circuit is further comprisedto detect noise on the cardiac signal and cancel classification of thecardiac response to pacing based on the detection of noise.
 73. Thedevice of claim 68, wherein the control circuit is configured to definea first classification window based on a timing of a cardiac signalfeature associated with a captured response and to define one or moreadditional classification windows relative to the first classificationwindow.
 74. The device of claim 68, wherein the control circuit isconfigured to compare the one or more characteristics to one or morereferences respectively associated with one or more types of cardiacresponses.
 75. The device of claim 74, wherein the control circuit isconfigured to adapt the one or more references.
 76. The device of claim68, wherein the control circuit is configured to classify the cardiacresponse to the pacing pulse as a captured response.
 77. The device ofclaim 68, wherein the control circuit is configured to classify thecardiac response to the pacing pulse as a non-captured response.
 78. Thedevice of claim 68, wherein the control circuit is configured toclassify the cardiac response to the pacing pulse as a near non-capturedresponse.
 79. The device of claim 68, wherein the control circuit isconfigured to classify the cardiac response to the pacing pulse as anon-captured response and an intrinsic beat.
 80. The device of claim 68,wherein the control circuit is configured to classify the cardiacresponse to the pacing pulse as a fusion/pseudofusion beat.
 81. Thedevice of claim 68, wherein the control circuit is configured toclassify the cardiac response to the pacing pulse on a beat by beatbasis.
 82. The device of claim 68, wherein the control circuit isconfigured to use the classification of the cardiac pacing response todetermine a pacing energy capture threshold.
 83. The device of claim 68,wherein the controller is configured to modify at least one of thefirst, second, and third classification windows.
 84. A medical devicefor classifying a cardiac pacing response, comprising: means fordefining a plurality of classification windows relative to andsubsequent to the pacing pulse, wherein the plurality of windowscomprise a first classification window predominantly associated with acaptured response, a second classification window predominantlyassociated with a fusion/pseudofusion response, and a thirdclassification window predominantly associated with a non-capturedresponse; means for sensing a cardiac signal following the pacing pulse;means for detecting one or more characteristics of the cardiac signalwithin one or more particular classification windows of the plurality ofclassification windows; and means for classifying the cardiac responseto the pacing pulse based on the one or more characteristics and the oneor more particular classification windows.
 85. The device of claim 84,further comprising: means for detecting noise on the cardiac signal; andmeans for canceling classification of the cardiac response based on thedetection of noise.
 86. A medical device for classifying cardiacresponses to pacing, comprising: means for delivering a sequence ofpacing stimulations to a heart; means for defining a plurality ofclassification windows relative and subsequent to each pacingstimulation, wherein the plurality of windows comprise; means forsensing cardiac signals following the pacing stimulations means fordetecting characteristics of the cardiac signals within particularclassification windows, means for comparing one or more of thecharacteristics of the cardiac signals to a captured response templateand an intrinsic response template; and means for classifying a cardiacpacing response as a fusion/pseudofusion beat if the one or morecharacteristics are not consistent with the captured response templateor the intrinsic response template.
 87. The device of claim 86, furthercomprising means for using the classifications of the cardiac pacingresponses to determine a pacing energy capture threshold.
 88. A medicaldevice for cardiac response verification, comprising: means fordelivering a pacing stimulation; means for defining a plurality ofclassification windows relative and subsequent to the pacingstimulation, wherein the plurality of windows comprise a firstclassification window predominantly associated with a captured response,a second classification window predominantly associated with afusion/pseudofusion response, and a third classification windowpredominantly associated with a non-captured response; means for sensinga cardiac signal following the pacing stimulation; means for detecting apeak of the sensed cardiac signal occurring within a particularclassification window; and means for determining a cardiac responsebased on the detected peak and the particular classification window.